From Bon Appétit.
Preheat oven to 375°F.
Butter 8-inch glass baking dish.
Mix both sugars, flour, and cinnamon in large bowl.
Mix in apples, then prunes.
Let mixture stand 5 minutes.
Transfer filling to prepared dish.
Bake until beginning to bubble at edges, about 20 minutes.
Mix first 4 ingredients in medium bowl.
Add butter; using fingertips, rub in until moist clumps form.
Rub in hazelnuts.
Scatter topping over filling.
Continue baking until topping is golden and juices are bubbling thickly, about 20 minutes.
Cool slightly.
Serve warm with ice cream.
