Toast the pistachios and walnuts with the butter in a skillet over medium-low heat for 3 to 4 minutes. Stir in the powdered allulose, cinnamon, and sea salt, then cool completely.
Heat the heavy cream and almond milk in a saucepan until steaming. Whisk the allulose and egg yolks in a bowl until smooth.
Slowly whisk about 1 cup of the hot cream mixture into the yolks. Pour the tempered yolks back into the saucepan and cook over medium-low heat until the custard lightly coats the back of a spoon.
Remove from the heat and whisk in the pistachio butter, vanilla extract, cinnamon, orange zest, and xanthan gum until smooth. Refrigerate for at least 4 hours.
Churn the cold pistachio custard until thick and creamy.
Add two-thirds of the nut crunch during the final minute of churning, then transfer the ice cream to a freezer-safe container.
Sprinkle the remaining nut crunch over the top and freeze for 2 to 4 hours before scooping.