Preheat your oven to 160°C (140°C for fan ovens) and line a cupcake tin with 12 fairy cake/mini cupcake cases.
In a medium-sized bowl, cream together the butter and sugar until soft and fluffy. Stir in the vanilla extract.
Mix in the flour, cocoa powder and eggs, a little bit at a time, until you have a smooth, thick cake batter. Use a splash of milk to help it come together if needed.
Divide the cake mix equally between the fairy cake cases and bake for 15-18 minutes, or until a toothpick comes out clean.
Transfer to a wire rack and let them cool completely.
Mix the icing sugar, cocoa powder and milk together to make the icing then spread a little bit onto each fairy cake. Top with sprinkles.
Let the icing set then enjoy!