Method

Grease and line the base of 2x20cm cake tins with baking parchment. Preheat the oven to 160°C/140°C fan.

Pour 60ml water into a mug and heat in the microwave for 1 minute. Place the water, chocolate and 50g sugar into a small saucepan. Stir on a low heat until the chocolate has melted and the sugar has dissolved. 

In a large mixing bowl, combine the remaining caster sugar with the plain flour, baking powder, salt and bicarbonate of soda. In another large mixing bowl, whisk together the egg yolks, vegetable oil and vanilla extract until combined. Beat in the dry mix a spoonful at a time, then use an electric whisk to combine until smooth.

In another large mixing bowl, whisk the egg whites and cream of tartar to stiff peaks using an electric whisk (with clean beaters). Fold the whisked egg whites into the mixture until just incorporated then split the mixture between two mixing bowls. Note, the egg yolk mixture is very firm at first and requires a good amount of egg white to become a light consistency. You may need to use an electric whisk to incorporate the egg whites to start with. 

Fold the chocolate mixture into one of the bowls of plain cake mix. Then divide both plain and chocolate cake mixes between the lined cake tins, ensuring that you make alternate layers. Use a dinner knife to create a ripple effect in the mixture. Place the cakes in the oven and bake for  50-60 minutes or until the cakes are springy to the touch and baked through.

As soon as the cakes are ready, turn them upside down on a cooling rack and leave to cool completely.

Meanwhile, to make the meringue buttercream frosting, melt the chocolate in a heatproof mixing bowl over a pan of simmering water. Once melted, allow to cool. 

Place the sugar and 85ml water in a small saucepan and bring to the boil on a high heat (keep a sugar thermometer handy to check the temperature, it needs to reach 120°C.) Add the egg whites to a clean free standing mixer bowl and whisk until light and frothy. Check the temperature of the syrup. When it’s ready, keep the mixer running and pour the syrup in a steady stream into the egg whites. Keep whipping the meringue until it’s cool (this can take up to 10 minutes) and add the vanilla extract. When the meringue is cooled, add the butter a spoonful at a time and beat until smooth. Stop whisking, then use a spatula to gently fold in the cooled melted chocolate a little at a time. This can be stored in the fridge until needed.

Once the cakes are cooled, use a palette knife to run around the edges of the cake to release them, then turn out onto a board. Trim off any excess on the tops of the sponge to level them for stacking. 

To assemble, dollop a little frosting onto a serving plate or board, and place the first sponge on top. Spread the top of that layer with frosting and sandwich the second layer on top of the first. Spread the frosting evenly over the top and sides, scraping off any excess with a palette knife. Sprinkle over the white chocolate shavings and serve.