Line 2 x 20cm/8” round cake tins with parchment and preheat the oven. Put the butter and caster sugar into a mixing bowl and beat together until light and fluffy.
Beat in one egg followed by a spoon of cocoa.
Beat in the second and third eggs, adding a spoon of cocoa after each one. Beat in the final egg.
Sieve the flour into the bowl then stir it into the mixture. Add the milk and vanilla and stir to create a batter that just drops off a spoon.
Divide the mixture between the two prepared tins and smooth the tops. Bake for 25–30 minutes.
Leave the cakes to cool in the tins for 5 minutes before turning out onto a wire rack and leaving to cool completely.
To make the filling, put the unsalted butter into a bowl and beat until it softens.
Sift the icing sugar into the bowl and mix until it is combined. Sift the cocoa into the bowl and beat that in too.
Spread the filling over one cold cake layer then place the other cake layer on top.
Put the chocolate and unsalted butter into a bowl and gently melt them together in a saucepan over a medium heat, stirring constantly.
Once melted, leave to cool, then add the golden syrup. Slowly pour the icing over the top cake, allowing it to run down the sides a little.