Preheat the oven to 160°C/gas mark 3. Grease and line a 1.5l loaf tin. Beat the butter and sugar together for 4-5 minutes until light and creamy. Beat in the flour, eggs, buttermilk, cocoa powder, orange zest, baking powder and bicarbonate of soda until smooth. Scrape into the tin and bake for 55 minutes until risen and a skewer poked into the centre comes out clean. Do not turn off your oven.
Cool the cake in the tin for 10 minutes and then lift out on to a wire rack and leave to cool completely. Slice the cake into 2cm thick slices. Using a bunny shaped cookie cutter (any shape is fine, but no more than 7cm in height), cut bunny shapes from the cake slices. You should have about 12 bunnies. Very carefully – those bunny ears are fragile – lay them out on a lined baking tray and transfer them to your freezer.
Move on to your vanilla cake. Clean your loaf tin and then grease and line again. Beat the butter and sugar together until light and creamy. Gradually beat in the eggs, adding 1 tbsp of the flour if the mixture begins to curdle. Beat in the flour, milk and vanilla extract until smooth.
Remove the bunny cake cut-outs from the freezer. Pour a thin layer of vanilla cake batter along the bottom of the loaf tin. Stack the rabbits together to make a long bunny log and then line them along the bottom of the loaf tin. Pour the remaining cake batter over the cake cut-outs so they are completely covered, smoothing the batter over the top with an offset spatula.
Bake for 1 hour 5 minutes or until a toothpick inserted in the centre comes out clean. Remove the cake from the oven and place on a wire rack to cool for 10 minutes. Remove the cake from the pan and let it cool completely. Slice up and serve.