Method...

1. 1. Pre-heat the oven to 180°C/350°F/Gas Mark 4. Grease and line a large tray 25cm x 40 cm.
2. 2. Mix the sugar and eggs in a large bowl for about 4-5 minutes, or until light and fluffy. Melt the butter in a small pan on a medium-heat, add the Camp Coffee and then fold it through the sugar and egg mixture with a spatula.
3. 3. Sieve the flour and cocoa powder into the bowl and carefully fold the mixture together, in a figure of 8 motion. Once combined, spread the cake mixture across your prepared tray so it is about 1cm thick. Cook for 10 minutes, or until firm and springy to the touch. Leave to cool.
4. 4. Meanwhile, break up the white chocolate and add to a glass bowl with the milk. Stir over a pan of gently simmering water, making sure the water does not touch the base of the bowl and leave to melt, stirring occasionally, until smooth. Remove from the heat and leave to cool slightly.
5. 5. Carefully mix the mascarpone with the caster sugar, egg yolk and sherry then fold through the melted white chocolate. Tear the cooled sponge into chunks and place in the base of a 20cm x 30cm serving dish in a double layer. Combine the Camp Coffee with the rum and water, and drizzle over the cake evenly. Leave to soak in for a few minutes.
6. 6. Spoon the mascarpone mixture over the top, spreading it evenly. Dust the whole thing with a little cocoa powder, and scatter with shavings of dark chocolate. Place in the refrigerator until you are ready to serve.