8 eggs, separated
500 grams (2 cups) castor sugar, divided (see Note*)
250 grams (2 cups) ground hazelnuts
300 grams (10 oz.) dark chocolate
250 grams (1 cup) unsalted margarine or butter
Beat egg whites until stiff. Slowly add half the sugar, beating continuously. Then fold in ground hazelnuts. Pour mixture into a 9 x 13-inch baking dish which has been lined with parchment paper; spread evenly in pan. Bake at 180°C (350°F) for 20 minutes. Remove from oven and let cool.
Melt chocolate; set aside and let cool. Cream margarine or butter with remaining 1 cup of sugar. Add egg yolks and blend well. Add melted chocolate. The mixture should be light and fluffy. Pour over the nutty meringue base and bake 20 minutes longer.
Remove from oven and let cool completely. Store in the freezer as it is much easier to cut into squares when frozen. Cut into squares. (N.B. If this is left out of the refrigerator for too long, the chocolate may begin to melt.)
Note: To make castor (superfine) sugar, process granulated sugar on the Steel Blade in the food processor for 1 to 2 minutes, until fine.