Preheat the oven to 180°C (160°C fan bake). Line a 12-hole muffin pan with paper cases.
Cream the butter and sugar until the mixture is light and fluffy.
Add the eggs, one at a time, beating well after each addition.
Whisk the flour and cocoa together in a small bowl.
Fold in some of the flour mixture to the creamed mixture, followed by some of the milk, then some of the flour. Repeat until all the flour and milk has been mixed in.
Spoon the mixture into the muffin pan.
Bake for 18 minutes or until a skewer pushed into the centre of a muffin comes out clean.
Leave to cool.
To make the icing, whisk the icing sugar and cocoa together. Mix in the softened butter and 1 tablespoon of boiling water. Mix, adding more water as necessary until you get an icing consistency.
Using a knife, spread a little icing over each cupcake. Decorate with whatever you like.
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