Toast the hazelnuts with the butter in a skillet over medium-low heat until fragrant. Stir in the powdered allulose and sea salt, then spread on a plate to cool completely.
Whisk the heavy cream, almond milk, allulose, and cocoa powder in a saucepan. Heat until steaming and completely smooth.
Whisk the egg yolks in a bowl. Slowly add about 1 cup of the hot chocolate mixture while whisking, then return everything to the saucepan.
Cook over medium-low heat, stirring constantly, until the custard lightly coats a spoon. Remove from the heat and whisk in the chocolate hazelnut spread, vanilla extract, and xanthan gum.
Refrigerate the base for at least 4 hours until thoroughly cold, then churn until thick and creamy.
Add half of the toasted hazelnut crunch during the final minute of churning. Transfer to a freezer-safe container and scatter the remaining crunch between layers.
Freeze for 2 to 4 hours for a firm but creamy scooping texture.