Line a 20 cm springform tin. Cream butter and sugar until fluffy. Add yolks one at a time, beating well after each addition. Add melted chocolate and vanilla. Set aside. Whip 6 egg whites until soft peaks form, stir 1/3 into the chocolate mixture and fold in the rest. Pour into tin and bake for 25min. at 180°C.
Mix nuts, chopped chocolate and cornflour. Whisk 4 egg whites with cream of tartar until soft peaks, add the sugar gradually until stiff. Fold in hazelnut mixture.
Remove cake from the oven, spread meringue on top and return to oven, bake 25 min or until meringue is crisp.
