Preheat oven to 400 °F and lightly butter a baking dish.
In a food processor, pulse ground hazelnuts and sugar. Add butter while pulsing, then add liqueur and vanilla, pulsing until a paste is formed. Stuff center of each apple with hazelnut mixture. Cut puff pastry into long strips, 1-inch wide. Starting at base of each apple, wrap apple with pastry so that pastry overlaps just slight as it spirals around the apple. Brush pastry with eggwash, sprinkle with Turbinado sugar and place in prepared dish. Bake for 10 minutes, then reduce oven temperature to 375 °F and bake for about another 15 minutes, until pastry is a rich golden brown.
For the Cardamom Custard Sauce:
Bring cream, cardamom and vanilla up to a simmer over medium low heat. In a small bowl, whisk egg yolks and sugar. Slowly pour warm cream over yolks while whisking and return mixture to pot. Stir with a wooden spoon over medium-low heat until the custard coats the back of the spoon. Strain sauce and chill until ready to serve.