Preheat the oven to 180°C, fan 160°C, gas mark 4. Grease a 12 hole muffin tin.
Beat the 225g of the butter with the caster sugar until light and creamy. Add the eggs, one at a time, mixing well.
Fold in the flour and stir through the 100ml of milk and 1 teaspoon of the vanilla extract until the mixture is smooth. Spoon half the mixture into another bowl and stir in the cocoa and remaining milk.
Add a large spoonful of both cake mixtures into each muffin tin, then drag a skewer (or teaspoon handle) through a few times to create a marbled effect. Bake for 25-30 minutes until springy, then leave to cool.
For the buttercream, beat the remaining softened unsalted butter and icing sugar together with an electric whisk until creamy. Add the remaining vanilla extract and beat again.
Use a piping bag to pipe onto the cakes. Sprinkle over the honeycomb pieces and wrap in cupcake frills for an extra decorative touch.