This is the recipe I use and will generously decorate 12-15 cupcakes: 250g Butter, 500g icing sugar, 2tbsp milk, 1tsp vanilla extract, 200g melted chocolate (dark, milk or white, they all work really well).
leave the butter out of the fridge for at least an hour before you start. Then, cream the butter using an electric whisk until it's really soft and the colour becomes lighter. Add the vanilla extract and milk, whisk again until fully combined. Melt the chocolate and let it cool for a few minutes. Slowly add the melted chocolate, whisking as you go. Once all the ingredients are combined sift in the icing sugar (fold in using a spoon first to avoid an icing sugar cloud covering the whole kitchen). Whisk again until light and fluffy.
I find using cocoa powder can give buttercream a bitter taste and much prefer to use real chocolate :-)
thats like the one I use doublenegativepanda to make fancy cupcakes - its got kind of cold white sauce in it!!...I think its called 'velvet buttercream' - if the cold custard/white sauce looks at all lumpy just press it through a sieve.