Polka Dot Slice

Published by Amy

Posts may contain affiliate links. Please see our Privacy Policy.

Easy and delicious tray bake, made with rainbow chocolate drops, coconut and vanilla. A family favourite but particularly loved by kids!

Polka Dot Slice recipe from Baking with Granny. Easy tray bake cake for kids, made with coconut and rainbow chocolate drops (such as Smarties or M&Ms).

Whenever I’m looking for inspiration for recipes, I go straight to my collection of old recipes books. Some from Granny (although those are very much on loan!) but most from Great-Granny. Some are proper publications, some are binders filled with recipes from magazines & newspapers, but many are little DIY books, made by groups who wanted to raise some money for whatever their cause. These are undoubtedly my favourite recipe “books”.

I first came across Polka Dot Slice in one of these lovingly made books, and I instantly knew it would be a hit with my kids. Anything containing some Smarties or M&Ms is guaranteed to please two little boys! It’s why our Rainbow Cookies are always a highly requested bake at home.

[adthrive-in-post-video-player video-id="k9RTwKaN" upload-date="2023-01-31T14:04:51.000Z" name="Polka Dot Slice | Baking with Granny" description="Easy and delicious tray bake, made with rainbow chocolate drops, coconut and vanilla. A family favourite but particularly loved by kids!" player-type="default" override-embed="default"]

As expected, the kids loved Polka Dot Slice! A lovely little tray bake cake, made with coconut, vanilla and chocolate rainbow drops. What’s not to love?

The texture is less “light & fluffy cake”, and more “perfect amount of bite with sweet delicious flavours”, thanks to the desiccated coconut adding an extra dimension of texture, not to mention taste.

As is often the case in recipes inspired by the ones that I find in these little old cook books, I’ve played around with the various elements of this sweet rainbow cake traybake to get everything just-right. My favourite addition? That sprinkling of sugar on the top, which finishes it off beautifully and adds just a little crunch too.

Polka Dot Slice recipe from Baking with Granny. A rainbow cake traybake, made with smarties, coconut and plenty of sugar.


Margarine or Butter
My personal preference is to use a margarine due to it being easier to cream with the sugar. However butter is perfectly fine to use too, if you wish. Just ben sure your margarine or butter is softened first, to make the creaming easier.

Caster Sugar
Polka Dot Slice is a cake tray bake, so we start with a cake-like ingredient base. Caster sugar is preferable because of this. You could swap out for a golden caster sugar, if you want aa slightly less-sweet option, or a more caramel-y flavour.

Free-range Eggs
Again, this is a cake tray bake, so the usual cake base of butter/margarine, sugar, eggs and flour applies. It isn’t too crucial what size of eggs you use in Polka Dot Slice; just be sure to use free-range when possible.

A little bit of liquid helps make this tray bake cake batter a little easier to work with, especially once you’ve added the coconut and flour. What kind of milk you use is up to you; dairy, soya, oat, coconut, almond…all should work fine in this recipe.

Vanilla Extract
I’ve specified vanilla extract as the vanilla of choice in this traybake but if you are feeling fancy you could swap it out for vanilla paste or a vanilla pod. Just avoid using a vanilla essence – the synthetic taste is never really comparable

Desiccated Coconut
The coconut not only adds another level of flavour to Polka Dot Slice, it also changes the texture from a classic sponge cake, to a slightly denser tray bake cake. Alongside the vanilla and the chocolate rainbow drops, the desiccated coconut becomes an underlying complimentary taste; as opposed to a full-on flavour at the forefront.

Smarties or M&Ms
Either is fine, it’s really a case of personal preference, or whichever you can get hold of easier. I usually opt for Smarties as they are easier to get hold of locally, but M&Ms do have a slightly brighter appearance.

Polka Dot Slice. Tray bake cake made with Smarties or M&Ms, perfect for kids.

Polka Dot Slice Questions

Can I swap out the Smarties/M&Ms for something else?
Sure, why not. It will obviously take away the rainbow polka dot finish but there’s no reason you can’t tailor the recipe to your personal taste. Some alternatives I think would work well could be chocolate chips, fudge chunks, chopped nuts or even dried fruits.

I don’t like coconut, what can I use instead?
We love coconut and think it is a must-have in this recipe. However, a possbile alternative ingredient that may work is ground almonds. I haven’t tried and testsed this but it could be worth a shot if you really don’t like coconut and still want to give this recipe a try

Polka Dot Slice recipe. Easy tray bake cake, perfect for baking with kids. Chocolate rainbow drops, with coconut and vanilla flavours.

Granny's Top Tips

• The recipe specifies you use approximately 80g of Smarties or M&Ms. The reason for this is because a tube of Smarties in the UK is usually 38g, and a bag of M&Ms 45g. I’ve tried to give an amount that will allow you to follow the recipe without having to buy unnecessary extras, or leave you short. You can of course use more or less to suit your taste, or to what you have readily available.

Polka Dot Slice recipe from Baking with Granny. Easy tray bake cake for kids, made with coconut and rainbow chocolate drops (such as Smarties or M&Ms).

Polka Dot Slice

Easy and delicious tray bake, made with rainbow chocolate drops, coconut and vanilla. A family favourite but particularly loved by kids!
4.75 from 4 votes
Course: Dessert, Snack
Cuisine: British, Scottish
Prep Time: 10 minutes
Cook Time: 30 minutes
Servings: 12



  • Pre-heat your oven to 180°c (160°c for fan assisted ovens or Gas Mark 4). Grease and line a 11x7 inch tray with some greaseproof paper. Set aside.
  • In a large bowl, cream together the butter/margarine and sugar until light and fluffy. Add the eggs one at a time, mixing through before adding the next. Add the milk and vanilla extract. Mix through to combine.
  • Add the coconut and mix through until combined.
  • Sift in the self-raising flour and fold together to create a batter.
  • Add the majority of the Smarties/M&Ms, holding back some (to top the Polka Dot Slice with before baking). Mix through until evenly distributed in the batter mixture.
  • Spoon the mixture into your pre-lined tray, spreading evenly to the edges. Top the cake with the remaining Smarties/M&Ms. Bake in your pre-heated oven for 20-30 minutes, until golden in colour, and a skewer inserted to the middle comes out clean.
  • Allow your Polka Dot Slice to cool for a few minutes before sprinkling a little extra sugar over the top of it, whilst still warm. Once completely cool, remove from the tray and cut into individual portions.
Tried this recipe?Tag @bakingwithgranny or use the hashtag #bakingwithgranny!


Nut-free: This recipe for Polka Dot Slice doesn’t contain any nuts (coconut isn’t a nut, despite the name) but be sure to double check each ingredient for any hidden allergens.

Dairy-free: To make your Polka Dot Slice dairy-free, simply use a dairy-free margarine, a dairy-free milk and Daisy & Dom D&Ds.

Polka Dot Slice recipe from Baking with Granny. Easy tray bake cake for kids, made with coconut and rainbow chocolate drops (such as Smarties or M&Ms).

8 Responses

  1. Tried this on my Grandchildren with ground almonds it was lovely They loved the smarties !
    Gosh Granny you are clever , I took all the praise !!
    It’s simple but different and tasty
    Thank you

  2. 5 stars
    Used ground almonds and some white chocolate smarties left over from Christmas and only just in date, Big Big hit with my grandbabies, especially the autistic one who doesn’t like dark or milk chocolate, and is often left out of things with smarties in, so many ranks for this, it will become a regular for me.

  3. 4 stars
    Made these twice – completely free from and gluten free flour and with the ‘normal’ ingredients. both times they tasted delicious but both times the smarties on top sunk into the mixture – so no Polka Dot effect. my mixture looked as the video. it certainly didn’t effect the taste and th chocolate was a surprise as no evidence on top!!!

  4. Delicious taste although the smarties sank to the bottom. Why do you think this happened. I will make them again. Many thanks

Leave a Review

Got a question? Tried this recipe & want to leave some feedback? Please use the comment section below! Required fields are marked *

Recipe Rating

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Hi! I'm Amy

Nice to meet you

Hello, I’m Amy, the voice-behind and creator-of Baking with Granny.

Subscribe For Recipes