This chocolate-hazelnut mousse can be served alone with whipped cream, or used as filling for delicious cakes.
Place both chocolates in a heat-proof bowl; set over a pan of simmering water. Stir occasionally until completely melted. Remove from heat; set aside in a warm place.
Place cream in the bowl of an electric mixer fitted with whisk attachment; whip until soft peaks form. Transfer to a medium bowl. Cover; chill. Wash and dry mixer bowl.
Place the egg yolks in the mixer bowl, and whip on medium speed until pale yellow, 4 to 5 minutes.
Place sugar, corn syrup, and 1 tablespoon water in a small, shallow saucepan set over medium-high heat. Attach a candy thermometer to side of pan, and cook until sugar syrup reaches soft-ball stage (238 degrees).
Increase mixer speed to high; pour sugar syrup down side of bowl; continue mixing on high 5 seconds. Reduce speed to medium; mix until bottom of bowl is warm to the touch. Quickly whisk egg mixture into chocolate. Whisk in one-third of chilled whipped cream; gently fold in remainder. Cover with plastic, and chill until set, about 2 hours.