Pre-heat your oven to 170°c and line your muffin tray with 8-10 cupcake cases.
Sift the flour, sugar & baking powder into a large bowl. Add the butter and mix on a medium speed until the mixture resembles a sandy consistency.
Gradually add half the milk to the mixture. Whisk the egg & syrup into the remaining milk and again gradually add to the other ingredients. Mix until only combined - do not over mix!
Using an ice cream scoop, fill your cupcake cases 2/3 full and bake for around 20 minutes until a skewer comes out the cupcakes clean.
When cool enough to touch, transfer to a wire rack to cool completely.
For the Icing
Sift the icing sugar into a large bowl, add the butter and mix on a slow-medium speed with a clean tea towel over your mixer to prevent your kitchen filling with icing sugar!
When the butter & sugar start to come together, gradually add your Irn Bru syrup a spoonful at a time. Continue to mix until completely combined and of a light & fluffy consistency. If you want a deeper orange colour then you can add a dab of orange food colouring also.
Pipe on to your cupcakes when they're completely cool and sprinkle with a little blue sugar if desired.
Instructions for creating your Irn Bru syrup can be found at Baking with Granny.