Grease a 2lb loaf tin and preheat the oven to 180 degrees C. Pour the cocoa powder into a mixing bowl then add the boiling water. Cream together until you have a thick paste.
Place the sugar and butter into the same bowl, and mix all the ingredients together until smooth. Next, add the milk and both eggs and mix all of the ingredients again until fully combined.
Gradually add the sifted baking powder, flour, orange zest and grated chocolate orange into the bowl and fold into the cocoa/egg mixture until fully incorporated.
Pour your chocolate mixture into the prepared loaf tin and place into the oven for 35-40 minutes, checking regularly. If your cake tester/knife comes out clean and the cake is baked through, take out of the oven and set aside to cool out of the tin and onto a wire rack.
As the cake cools begin to make your icing by melting both chocolates in a microwave, checking every 10 seconds. Once completely melted, set aside. Cream together the butter and icing sugar in a mixing bowl until light and fluffy with no lumps then gradually add the melted chocolate, stirring continuously until you have a smooth and creamy buttercream.
Place your cake onto a serving plate and top with a generous amount of icing and some leftover chocolate orange segments (if you havent scoffed them). The cake will keep in an airtight container for 3 days.