Preheat oven to 140 C, 275 F, Gas Mark 1. Line 2 baking sheets with non-stick baking parchment and draw a 23cm (9 inch) circle onto each sheet. Whisk the egg whites in a bowl until stiff but not dry. Gradually whisk in the sugar, a tablespoon at a time, whisking well between each addition. Then carefully fold in the chopped hazelnuts and chocolate. Spoon the meringue onto the circles, then spread neatly into rounds. Bake for approximately 1 1/2 hours or until dry and the undersides are firm when tapped. Turn off the oven and leave to cool in the oven. To make the filling: In a bowl combine the mascarpone with the cream to make a loose mixture, spread over the base of one of the meringue discs and sandwich together with the remaining disc. Dust with cocoa powder and serve. NOTES : A wonderful combination of hazelnut and chocolate meringue sandwiched together with creamy mocha flavoured mascarpone.