Beat butter and sugar untill is fluffy,and add egg one at a time and continue beating.and the flavour

Sift flour and baking powder,add the flour and baking mixture in batches.

Then add milk increase speed of mixer.

Get a clean bowl and add your icing sugar and margerine
Mix with you electric mixer
Add water, flavour, colour of your choice and mix until mixed
Prepare your pipping bag, add your Buttercream and decorate your cake