Step 1Heat a convection oven to 400 degrees.
Step 2In a bowl set over a pan of simmering water (double boiler), melt the butter and chocolate. While the butter and chocolate are melting, in a separate bowl, whisk together the sugar and cornstarch, along with the eggs and egg yolks.
Step 3Whisk the melted chocolate mixture into the egg mixture. Divide the mixture among 6 (10-ounce) ramekins, filling each with a generous 1/2 cup mixture.
Step 4Place the ramekins onto a baking sheet (this can be done in 2 batches), and bake just until the mixture begins to souffle and the top of each has mostly crusted over (there will be a "skin" in the very center that is not quite hard), 10 to 12 minutes depending on the oven.
Step 5Top each ramekin with a sprinkling of toasted hazelnuts and a scoop of ice cream before serving.