In the bowl of a food processor, add the flour and sugar and process a few times to mix together. Next add the butter and pulse a few times until the mixture looks like wet sand. Add the egg and yolk and process a few seconds more until the dough forms . Be sure to not over process the dough.
Dump the dough from the food processor bowl onto a lightly floured counter. Form the dough into four disks and chill in the refrigerator for about an hour.
Preheat oven to 375°F with a foil-lined large baking sheet on middle rack.
When the dough is nicely chilled, remove from the refrigerator. Roll out the first piece of dough between 2 sheets of plastic wrap into a 1/8-inch thick or slightly thicker. Keep other pieces of dough chilling while assembling the tarts one at a time.
Spray a mini fluted tart pan with removable bottom with baking spray. Gently wrap the dough over the rolling pin and place it over the first mini tart pan (or an 11-inch fluted tart pan); release the rolling pin and let the dough fall into the tart pan. Press the dough softly into the bottom of the tart pan. If some of the dough breaks while you press it into the tart pan, that’s not a problem. Simply piece the dough together. Press the sides of the dough about 1/2 -inch up the side of the tart pan. The rim of the tart should be lined with a slightly thicker layer of pastry than the bottom (about 1/4-inch thick). Cut off the extra dough from the sides and keep these excess pieces to make the lattice topping. Repeat this process with the remaining 3 tart shells.
Wrap the remaining dough in plastic wrap. Place the remaining dough and the tart pans in the refrigerator for about 30 minutes to chill.
When the tart shells are chilled, remove them from the refrigerator and prick the pastry bottoms with a fork a few times. Fill each tart shell with Nutella almost all the way to the top. Roll out the extra dough into lattice strips. Place the dough strips in a criss-cross pattern over the Nutella. Repeat the same process with the remaining three tart shells.
Place the mini crostatas on a baking sheet and bake for about 25-30 minutes at 375 F, or until the crust is a light golden brown. *Check the crostata at 25 minutes. Every oven is different. The crust should not be too brown.