Cupcake: Preheat the oven to 180 degrees C. Line a 12 hole muffin tin with paper cases.
Put the butter and sugar into a bowl and whisk until light and fluffy. Gradually add the eggs, beating between each addition.
Sift the flour into the mixture. Using a large metal spoon, gently fold in the flour until well combined. Stir in the vanilla and milk. The mixture should easily drop off a spoon. Divide the mixture between the cases.
Bake for 20 minutes in the preheated oven, or until a knife comes out clean when inserted into a cake.
Buttercream Icing: Put the butter in a large bowl and beat until creamy. Sift in the icing sugar and cocoa, then whisk until well combined.
When the cakes are completely cool spread the icing over with the back of a teaspoon then add your decorations.