Step 1

Preheat the oven to 180°C/350°F/gas

Step 2

Grease a 25cm x 30cm roasting tray and line with a sheet of damp greaseproof paper. In a food processor, blitz 250g each of butter and icing sugar, then crack in the eggs, add the flour, baking powder, 50g of cocoa, and a splash of juice from the mandarin tin, and blitz again until smooth. Use a spatula to gently spoon the mixture into the tray in an even layer, and bake for 20 minutes, or until risen and an inserted skewer comes out clean. Remove from the oven, leave for 5 minutes, and lift out on to a wire rack to cool completely.

Step 4

Meanwhile, make the buttercream. In the processor, blitz the remaining 200g of butter and 400g of icing sugar until pale and fluffy, then pulse in the cream cheese, loosening with a splash of mandarin juice, if needed. Drain the mandarin segments and place on kitchen paper. On a serving board, carefully cut the cool cake through the middle to give you two large flat rectangles (I like to use a bread knife and run it under the hot tap first to help get a smooth cut). Spread a third of the buttercream over one of the cakes, dot over the mandarin segments, and sit the other cake on top. Trim the edges for a smart finish, if you like (you can call the offcuts chef's treat!). Pulse the last 25g of cocoa into the remaining buttercream, and use it to decorate the top of the cake, making dips and peaks with the back of your spoon.