Prepare an ice bath and set aside. Gather 4 large or 6 small martini glasses (or ramekins if you do not have martini glasses). Set aside.

Combine the corn syrup and water in a small bowl and whisk until smooth.

Blend the hazelnuts and powdered sugar into a paste in a food processor or blender. Set aside.

Beat the cream and granulated sugar until soft peaks form. Set aside.

Combine the sugar syrup, egg yolks, Marsala, and Frangelico in a metal bowl. Place over a pot of boiling water and whip with an electric mixer until the sauce is very thick and forms thick ribbons when you turn off the mixer and lift the beaters out of the mixture . Remove from the heat and continue to whip until the mixture has cooled. Carefully fold in the vanilla and whipped cream until just combined. Fold in the hazelnut paste until evenly incorporated. Do not overmix.

Divide the mixture among the martini glasses, cover with plastic wrap, and freeze for at least 3 hours or overnight.

Thirty minutes before serving, transfer the glacees to the refrigerator to soften. Dust with cocoa powder or powdered sugar, if desired, and serve.