Grease a 10-inch spring form pan and line with parchment paper.
In a food processor grind the hazelnuts and 3/4 cup sugar together, pulsing to avoid over blending so it does not become a paste. Whip the cream using a mixer fitted with a whisk attachment or a hand mixer, until fluffy, soft peaks. Transfer to the refrigerator.
In a clean dry bowl, whip the egg whites until soft peaks form. Add the vanilla extract and 1/4 cup sugar and continue whipping until glossy and stiff, about 30 seconds more. Fold into the whipped cream, then fold in the ground nut mixture. Spoon the mixture into the spring form pan. Smooth the top. Freeze at least 4 hours or overnight.
Warm Glossy Chocolate Sauce: In the top half of a double boiler, combine the 2 chocolates over simmering water. Stir constantly until melted, then whisk in the corn syrup and water. Whisk until smooth and shiny.
When ready to eat, remove the collar and serve frozen with chocolate sauce.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.