Preheat the oven to 180 degrees C. Sift the flour and sugar into a bowl. Add the eggs and margarine and mix until everything is combined and is smooth.
Add 1 tsp vanilla essence and mix until combined. Equally separate the mixture into 12 cupcake cases, and bake in oven for approximately 12 minutes.
When they are finished, they should be golden brown on the top and spring back up when touched. Whilst waiting for the cupcakes to cool, start preparing the buttercream icing.
Sift the icing sugar into a bowl and mix in the butter - this may take a while if the butter isn't softened enough. Once completely combined, apply generously on top of the cupcakes. You can add whatever toppings and sprinkles you like, or just leave them as they are.