Preheat your oven to 180℃ (160℃ for fan ovens) and line an 8x10-inch baking tin.
In a large bowl, cream together the butter and sugar then stir in the vanilla extract. An electric hand whisk is handy for this.
Whisk in the eggs, flour, cocoa powder and milk, a little bit at a time, until you have a smooth cake batter.
Transfer the batter to your lined tin and spread it out evenly. Bake for 25-30 minutes, until a toothpick comes out clean.
Let the cake cool in the tin for around 10 minutes then flip it out onto a wire rack and leave to cool completely.
Icing and sprinkles
Mix the icing sugar and cocoa powder together then stir in the milk. To make your icing thinner, add more milk, and to make it thicker, add more icing sugar.
Spread the icing on top of the cake then top with the chocolate sprinkles.
Leave the icing to harden then cut into slices and enjoy!