Preheat the oven to 160°C.
Grease two 23cm cake tins with butter and dust with cocoa powder.
Sift flour, bicarbonate of soda, baking powder, and cocoa into a bowl and stir in sugar.
Melt butter and milk in a saucepan and let cool slightly.
Beat syrup, eggs, and the milk mixture into the dry ingredients to form a smooth batter.
Bake in the tins for 40-50 minutes, then cool completely on a wire rack.
Increase oven temperature to 170°C and grease one 23cm tin.
Sift flour, baking powder, and bicarbonate of soda for the orange sponge.
Cream butter and sugar for 5 minutes until pale.
Beat in eggs one at a time, adding flour if the mixture curdles.
Fold in orange zest, flour mixture, and orange juice.
Bake the orange sponge for 45-50 minutes until a skewer comes out clean.
Cool the orange cake for ten minutes in the tin before moving to a wire rack.
Heat cream in a pan until just under the boil.
Pour hot cream over chopped chocolate and stir until smooth and thickened.
Whisk cream to soft peaks and fold in orange zest.
Place one chocolate cake layer on a plate and spread with half the orange cream.
Add the orange cake layer, remaining cream, and final chocolate layer.
Cover the entire cake with chocolate ganache using a palette knife.
Roll fondant to 2mm, cut a 23cm circle, and decorate with stencil and gold dust.