How to Make It
Preheat the oven to 375° and line a baking sheet with parchment. Spread the nuts in a pie plate and roast for 12 minutes, until the skins split. Transfer the nuts to a clean kitchen towel and let cool. Rub them together to remove the skins and transfer to a food processor. Add the confectioners' sugar and flour; process until the nuts are finely ground.
In a large bowl, using an electric mixer, beat the egg whites with the salt at high speed until soft peaks form. Beat in the granulated sugar, 1 tablespoon at a time, until the whites are stiff. Using a rubber spatula, fold in the ground nuts. Spread the batter on the parchment in a 9-by-12-inch rectangle. Bake for 20 minutes, until golden. Prop the oven door open and let the dacquoise sit for 1 hour.
Meanwhile, in a microwave-safe bowl, melt the white chocolate in the microwave at high power in 1-minute intervals. Stir in the Nutella and crushed ice cream cones.
Invert the dacquoise onto a surface and peel off the paper. Spread evenly with the Nutella filling. Cut the dacquoise in half to make two 9-by-6-inch rectangles. Freeze until the filling is set. Spread the ice cream over one half. Close the sandwich and freeze until firm, at least 4 hours. Slice and serve.