Preheat your oven to 160 C / Gas 3. Prepare a 20cm push through cake tin by lining with greaseproof paper. Ensure the greaseproof paper comes over the top of the tin by 30mm to cook the cake evenly and stop the cake burning in the oven.
Cream the 125g butter and sugar with an electric mixer until creamy.
Sift the flour, cocoa powder, baking powder and bicarbonate of soda into a separate bowl.
Gradually add beaten egg, the flour mix and the buttermilk into the butter mixture until all have been added and the mix is smooth. Fill the lined cake tin.
Bake in the centre of the oven until a skewer comes out clean when pricked, about for 45 minutes.
Remove from oven and turn out onto a wire rack to cool for at least 2 hours, preferably overnight.
For the frosting, melt the dark chocolate, milk chocolate and 250g butter over a saucepan of boiling water.
Once fully melted, beat in with a wooden spoon, the icing sugar (Do not whisk otherwise you will cream the icing sugar and butter and the frosting will set too quickly).
Leave the frosting to chill and set for about 1 hour until thickened.
Slice the cake into 3 layers with a very sharp knife or cutting wire.
Smear the frosting on each layer of the cake and piece together. Smear frosting over the cake in its entirety until fully covered.
To make a chocolate orange cake, add a spoonful of orange essence to the cake mix and the zest of half an orange to the frosting.