Generously grease a 33x22.9cm (13x9in) traybake cake tin with butter and set aside.
In a large bowl, melt the marshmallows in 30 second bursts in the microwave.
Add the butter/margarine and mix through, before adding the rice krispies & roughly chopped mini eggs. Mix all the ingredients together.The mixture will be incredibly sticky - if it starts to set and becomes unworkable at any stage, pop it back in the microwave for a further 10 seconds.
Transfer mixture to your pre-greased tin and press into tin with the back of a spoon.
To top with Mini Eggs; push the eggs into the top of your Rice Krispies Squares before they set.
Leave to set for a few hours and cut into individual squares.