Preheat the oven to 180°C/160°C Fan/Gas 4 and line a 30 × 23 (12 x 9in) traybake tin with non-stick baking paper.
To make one traybake, measure the cocoa powder and boiling water into a large bowl. Stir to mix to a smooth paste. Add all the remaining ingredients and whisk using an electric whisk for about 2 minutes until light and fluffy.
Spoon into the tin and level the surface. Bake in the preheated oven for about 35 minutes, until well risen and shrinking away from the sides of the tin. Leave to cool in the tin on a wire rack, then remove the baking paper.
Repeat with the same ingredients and method to make a second traybake. (If you have two traybake tins, you can bake them at the same time.)
While the cakes are cooling, make the ganache. Pour the cream into a saucepan and heat over a medium heat until hot. Remove from the heat and add the chocolate. Stir until melted. Set aside in a cool place and, as it cools, it will thicken into a spreadable icing that will hold its shape when piped.
Place one cake on a serving board and drizzle with half the cream liqueur. Fold the remaining cream liqueur into the whipped cream and spread on to the cake on the board. Place the second cake on top and press down gently.
Put a fluted star nozzle into a piping bag and fill the bag with 350g (12oz) of the chocolate ganache and set aside.
Spread the remaining ganache over the sides and surface of the cake, so it is completely covered. Using a fork, fork the sides to make a pretty horizontal pattern.
Using the ganache in the piping bag, pipe small stars around the top edge of the cake to create a border. Arrange the Maltesers in a line widthways next to the piped edge. Pipe another line of stars, then arrange a neat line of chopped truffles. Continue to pipe and arrange the chocolates in neat rows until the surface of the cake is completely covered.
Light some candles and cut into squares to serve.
Mary’s Tips: Can be made, assembled and piped up to 6 hours ahead. The cakes can be made a day ahead, ready to ice on the day. Freezes well iced. Use around 40% cocoa solids chocolate.
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