Preheat oven to 160ºC / 180ºC fan/Gas Mark 3.
Place butter and sugar into a bowl and beat until light, fluffy and pale in colour.
Add eggs and flour and combine all the ingredients together.
Separate the mixture evenly into three bowls and add purple, pink and blue food colouring to each.
Using a tbsp or ice cream scoop, spoon each colour into the tin randomly and swirl with a toothpick for a marble effect. Bake for 25-30 minutes until well risen and firm to touch.
Leave to cool in the tin for a few minutes then turn out onto a cooling rack. Leave to cool completly.
Beat the the butter, sifted icing sugar and vanilla flavour together until smooth to make the buttercream..
Trim and level your cakes. Layer each sponge using buttercream.
Add the black food colouring to the remaining buttercream and coat the cake.
Decorate with sprinkles and enjoy!
Tag @pmecake so we can see your amazing cakes!
To avoid a wonky cake, make sure your layers are neatly trimmed. For a straight top use a cake leveller and for the sides place a cake board slightly smaller than the cake and trim off the cake crust using a serrated knife!
Test bake your colours with a cupcake to help you decide on a shade of colour!