Grease the pan, line the base with paper and grease the paper. Heat the oven to 300°F/150°C.
Grind the walnuts with the chocolate in two batches in a food processor or a few at a time in a blender. Alternatively, grind them with a rotary cheese grater.
Cream the butter with three-quarters of the sugar. Beat in the egg yolks, one by one, then stir in the chocolate mixture. Stiffly whip the egg whites, add the remaining sugar and continue whipping for about 30 seconds to make a light meringue. Fold the meringue into the chocolate mixture in three batches.
Spoon the batter into the prepared pan and smooth the top. Bake for 45-55 minutes until the cake shrinks slightly from the sides of the pan and a skewer inserted in the center comes out clean. Let the cake cool in the oven 5-10 minutes as it is delicate. Let it cool completely in the pan before unmolding it.
Shortly before serving, mix the confectioners’ sugar and cocoa and sprinkle the mixture over the cake.