1. Preheat the oven to 300°F. Grease and line two 8-inch layer cake pans with parchment paper. Cream the butter and superfine sugar together until soft and fluffy. Gradually add the beaten eggs, mixing well after each addition. Blend the cocoa powder with the water. Add alternately with the flour into the butter mixture until all is blended smoothly. Divide the batter between the two pans and bake in the center of the oven for 25 minutes until firm and just coming away from the side of the pans. Cool on a rack. (At this stage the cakes can be frozen for up to a month.)
2. To make the icing, beat the butter and confectioners’ sugar together until light and fluffy. Add the cocoa powder and milk and beat until combined. Sandwich the cakes together using half of the icing, then use the remainder to top the cake. Decorate with a simple dusting of cocoa powder.
3. For the coulis, blend the raspberries, lemon juice, and sugar until smooth, then strain to remove any seeds.
Tip: To make chocolate curls to decorate the cake, melt 3 1/2 ounces of dark chocolate (70 percent cocoa solids), then pour onto a flat surface (such as a baking sheet) and spread it out thinly. Cool until set, then carefully drag a sharp knife from one end of the chocolate to the other to form curls.