From Anne's Food
This makes enough for about 30 mini-muffins or 15 normal-sized ones. It doubles easily.
125 ml milk
2,5 tbsp flour
1/2 tsp vanilla extract
120 g butter, at room temperature
120 g sugar
Whisk milk and flour in a small saucepan and heat until it thickens. Cook for a few minutes - it will thicken quite a lot! Remove from heat and let it cool completely. Add the vanilla.
Beat butter and sugar until pale and fluffy. (You probably want to use a stand mixer for this - but handheld beaters will work as well.) Add the milk/flour-mixture, which must be absolutely room temperature and not at all warm.
Mix until your frosting looks like whipped cream. Spread onto your muffins or cake.