Mix flour and sugar in a food processor a few times. Add butter and pulse until it resembles wet sand. Add the egg and yolk and process for a few seconds to form dough. Do not overprocess dough.
2
Dump the dough from the food processor onto a lightly floured counter. Make four dough disks and refrigerate for an hour.
3
Preheat oven to 375°F with a foil-lined large baking sheet on middle rack.Take chilled dough out of fridge. First, roll the dough to 1/8-inch or thicker between two plastic wrap sheets. Build tarts individually while chilling dough.
4
Spray a removable-bottom mini fluted tart pan with baking spray. Wrap the dough around the rolling pin and place it over the first mini tart pan (or 11-inch fluted tart pan); release the pin and let the dough fall into the tart pan. Press dough into tart pan bottom gently.
5
Do not worry if some dough breaks when pressed into the tart pan. Put the dough together. Press dough 1/2 inch up tart pan sides. Tart rim pastry should be 1/4-inch thicker than bottom pastry. Reserve extra side dough for lattice topping. Use the remaining 3 tart shells.
6
Plastic wrap the remaining dough. Chill the remaining dough and tart pans for 30 minutes. Remove chilled tart shells from the fridge and fork-prick the pastry bottoms. Pack each tart shell with Nutella almost to the top.
7
Make lattice strips from extra dough. Cross Nutella-covered dough strips. Repeat with three more tart shells.
8
Bake mini crostatas on a baking sheet for 25–30 minutes at 375 F until the crust is light golden brown.Check crostata at 25 minutes. All ovens are different. Avoid overbrowning the crust.