enough for one cake.

1. 100g unsalted butter, softened
2. 25g cocoa powder
3. 1/4 tsp vanilla extract
4. 100g icing sugar
5. 1/2 tbsp milk (only if necessary)

Beat the butter with an electric mixer until creamy. Add the rest of the ingredients, bar the milk. Beat at high speed until fluffy. If too thick, add the milk gradually until the icing is right.