Preheat the oven to 180 degrees Celsius, and line the tray bake tin with nonstick baking paper. Combine the cocoa powder and boiling water in a large bowl, stirring to mix. Add in remaining ingredients and whisk using an electric whisk for 2 minutes until light and fluffy.
Pour the mix into the tin and smooth out the surface. Bake in the oven for 35 minutes until cooked through. Leave to cool on a wire rack. Repeat with the same ingredients to make a second traybake (or you can bake two at the same time if you have two tins).
While the cakes cool, pour the cream into a saucepan over medium heat. Once hot, remove from the heat and add in chocolate. Stir until melted, then set aside in a cool place. It will thicken as it cools, becoming a spreadable icing.
Place one cake on a serving board and drizzle with cream liqueur. Fold the remaining liqueur into the whipped cream, and spread on the cake. Set the second cake on top.
Spready ganache icing over the cake until covered, leaving some to put in the piping bag. Pipe ganache around the cake to decorate as you please.
For the filling
- 4 tbsp cream liqueur (such as Baileys, Kahlúa or Amarula)
- 300ml (½ pint) pouring double cream, whipped to soft peaks
