Raspberry Chocolate Boxes
7 ounces semi-sweet chocolate, broken into pieces
3/4 cup fresh raspberries, to decorate
2 teaspoons unsalted butter, for greasing
1/4 cup superfine sugar
To make the mocha mousse, melt 2 ounces of the chocolate in a heatproof bowl set over a saucepan of gently simmering water. Add the coffee and stir over low heat until smooth and then remove from the heat and cool slightly. Stir in the egg yolk and the Kahlua. Whisk the egg whites in a separate bowl until stiff peaks form. Fold into the chocolate mixture and cover with plastic wrap and chill for 2 hours, until set.
For the sponge cake, lightly grease an 8-inch square cake pan and line with parchment paper. Put the egg and extra egg white with the sugar in a heatproof bowl set over a saucepan of gently simmering water. Whisk over low heat for 5-10 minutes, until pale and thick. Remove from heat and continue whisking for 10 minutes until cooled and a trail is left when the whisk is dragged across the surface.
Preheat the oven to 350F. Sift the flour over the egg mixture and gently fold it in. Pour the mixture into the prepared pan and spread evenly. Bake in the preheated oven for 20-25 minutes, until firm to the touch and slightly shrunk from the sides of the pan. Turn out onto a wire rack to cool, then invert the cake, keeping the parchment paper in place.
To make the chocolate boxes, grease a 12x9-inch jelly roll pan and line with wax paper. Place the remaining chocolate in a heatproof bowl set over a saucepan of gently simmering water. Stir over low heat until melted, but not too runny. Pour it into the jelly roll pan and spread evenly with a spatula. Set aside in a cool place for about 30 minutes, until set.
Trim the crusty edges off the sponge cake and then cut it into 12 slices, measuring 3x1 1/4 inches. Spread a little of the mocha mousse along the sides of each sponge rectangle and press 2 long and 2 short chocolate rectangles in place on each side to make boxes. Divide the remaining mousse among the boxes and top with raspberries. Chill until ready to serve.