Preheat oven and line traybake. 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.
For one traybake. Combine cocoa powder and boiling water in a large bowl, stirring to form a smooth paste. Add all remaining ingredients and whisk with an electric whisk for about 2 minutes until light and fluffy.
Bake. Spoon the batter into the tin and level the surface. Bake in the preheated oven for about 35 minutes until well risen and pulling away from the sides of the tin. Let it cool in the tin on a wire rack and then remove the baking paper.
For the second traybake. Repeat the process with the same ingredients to make a second traybake (if using two traybake tins, you can bake them simultaneously).
For the ganache. While the cakes cool, prepare the ganache. Heat cream in a saucepan over medium heat until hot. Remove from heat and add chocolate, stirring until melted. Allow it to cool, thickening into a spreadable icing that holds its shape when piped.
To assemble. Place one cake on a serving board, drizzle with half the cream liqueur. Fold the remaining cream liqueur into whipped cream and spread it on the cake. Place the second cake on top and press down gently.
To decorate with ganache. Insert a fluted star nozzle into a piping bag and fill it with 350g (12oz) of the chocolate ganache, setting it aside. Spread the remaining ganache over the sides and surface of the cake, completely covering it. Use a fork to create a decorative horizontal pattern on the sides. Using the ganache in the piping bag, pipe small stars around the top edge to form a border. Arrange Maltesers in a line widthwise next to the piped edge. Pipe another line of stars, then arrange a neat line of chopped truffles. Continue piping and arranging chocolates in neat rows until the cake's surface is completely covered.
Candles. Light candles, and when ready to serve, cut into squares.