Preheat oven to 180C. Melt butter and chocolate in a bowl over a saucepan of simmering water, then stir in egg yolks, sugar and hazelnuts, and set aside. Whisk eggwhites to soft peaks, fold into chocolate mixture and divide among four 180ml ramekins. Bake until puffed and just set (15-20 minutes). Serve hot with ice-cream.