Preheat your oven to 180C/160C Fan - grease and line your 2lb loaf tin!
Beat together your unsalted butter and sugar together until light and creamy!
Add in the flour and eggs and beat again until combined. It'll be a smooth cake mixture!
Split the mixture into two bowls, and add the vanilla extract to one, and the cocoa powder to the other!
Add the mix into the cake tin randomly with spoons and swirl together. Bake the cake for 55-65 minutes - but check from 50 minutes onwards.
Once the cake has baked, leave in the tin for 10-20 minutes, and then cool fully on a wire rack!
Beat the room temperature unsalted butter on its own for a few minutes to loosen and soften.
Add the icing sugar and beat together until light and fluffy!
Split the mixture between two bowls, and add the vanilla extract to one and the cocoa powder to the other!
Mix well - if its too stiff, add 1-2tbsp of boiling water to the buttercreams to loosen.
Add the buttercreams to a piping bag - I get a large piece of clingfilm and add the two buttercreams in two lines and wrap this up into a sausage. You then snip off the end, and add it to a piping bag with your favourite piping tip!
Pipe the buttercream onto the cake and add some sprinkles of choice!