Preheat the oven to 300*F. In a bowl placed over a pan of simmering water, melt the chocolate with the butter. Beat the sugar with the egg yolks until smooth. Mix this with the chocolate away from the heat, adding it gradually. Beat the egg whites in a clean bowl with a tiny pinch of salt until stiff. Fold into the chocolate mixture gently but thoroughly. Have ready two greased 7-inch cake tins.

Divide about two thirds of the cake mixture between the pans and bake in the oven for 1 hour. Meanwhile, keep the remaining chocolate cake mixture/mousse in the fridge. Let the cakes cool. Spoon the mousse over the cake base and sandwich the two halves together. Eat warm or cold.