Preheat the oven to 350 degrees F and line and grease a 1 1/2 pound loaf pan.
Cream together the egg yolks and sugar. Whisk the egg whites until very stiff. Add the grated chocolate and flour to the yolk mixture, then fold in the whites. Pour the chocolate mixture into the prepared loaf pan. Bake for 15 to 20 minutes. Turn out and cool on a rack. When cold, slice in half lengthways through the middle.
Spread both halves of cake with jam on the cut sides.
For the filling, mix together the cocoa powder, vanilla essence to taste, and whipped cream; spread evenly over one half on top of the jam.
Make the icing by melting the chocolate with the butter. Allow to cool slightly, then spread over the jam on the other half. Put the two strips together so that the icing and filling are together in the middle, with the iced strip on top, and chill slightly before slicing to serve.
Recipe courtesy of Jennifer Paterson and Clarissa Dickson Wright