Preheat the oven to 180°C.
Mix the cocoa and boiling water in a large bowl until smooth.
Beat the butter into the cocoa mixture until smooth.
Add remaining cake ingredients and mix until well combined.
Pour into the prepared tin and bake for 30 to 35 minutes.
Cool for 10 minutes in the tin then turn out and remove parchment.
Brush the warm cake top with jam and allow to cool fully.
Heat cream in a pan, add milk chocolate, and stir until smooth.
Spread the thickened chocolate icing over the cooled cake.
Melt white chocolate and pipe lines 1cm apart over the icing.
Drag a cocktail stick through the lines in opposite directions to feather.