1. Prepare the Chocolate Sponges:

1. Preheat the Oven: Set to 180°C (160°C Fan) / Gas 4. Line two 30 × 23 cm (12 x 9 in) traybake tins with non-stick baking paper.
2. Make the Cake Batter: In a large bowl, mix cocoa powder with boiling water until smooth. Add caster sugar, baking spread, self-raising flour, baking powder, eggs, and milk. Whisk using an electric mixer for about 2 minutes until light and fluffy.
3. Bake: Divide the mixture evenly between the prepared tins and smooth the tops. Bake for about 35 minutes until risen and pulling away from the sides.
4. Cool: Let the cakes cool in the tins on a wire rack, then remove the baking paper.

2. Prepare the Ganache Icing:

1. Heat the Cream: In a saucepan over medium heat, heat the double cream until hot but not boiling.
2. Melt the Chocolate: Remove from heat, add the plain chocolate pieces, and stir until fully melted and smooth. Let cool to a spreadable consistency.

3. Assemble the Cake:

1. Layer the Cake: Place one sponge on a serving board. Mix vanilla extract or chocolate syrup (if using) into the whipped cream and spread it over the sponge.
2. Stack the Cake: Place the second sponge on top and press down gently.

4. Decorate the Cake:

1. Frost with Ganache: Spread the ganache over the top and sides, smoothing it evenly. Use a fork to create a horizontal pattern on the sides.
2. Pipe Decorations: Fill a piping bag with 350g of ganache fitted with a fluted star nozzle and pipe a decorative border around the top.
3. Add Maltesers and Truffles: Arrange Maltesers in a neat line next to the piped border, followed by a line of piped stars and a row of chopped truffles. Repeat the pattern across the top of the cake.
4. Final Touches: Light some candles, slice, and serve!