In food processor pulse together flour, cornstarch, sugar and salt.
Add butter and pulse until mixture forms fine crumbs
Add water, about 1 Tbsp at a time until dough begins to cling together (you may need an additional Tbsp or 2 of water).
Pour crumby dough onto parchment paper and press together using the palm of your hand (flattening the crumbs and pressing together). Fold dough over onto itself and then repeat the process of flattening the dough with the palm of your hand (this helps create a flakier crust).
Divide into 8 equal-sized pieces (about 1/4 cup each), roll each into a ball, and tightly wrap with clear wrap.
Transfer to refrigerator and refrigerate for at least one hour.
Preheat oven to 400F
(if galette crust has been refrigerated for longer than one hour, you may need to let the dough sit at room temperature for 10 minutes before it can be easily rolled out).
In skillet, combine brown sugar, heavy cream, and butter over low heat. Stir until butter is completely melted.
Add apples, increase heat to medium-high. Stir constantly until mixture begins to boil and apples are tender.
Remove from heat, stirring occasionally for 5 minutes while mixture cools. After 5 minutes, stir in vanilla and salt.
Allow mixture to cool while you prepare galette crust.
Line a large cookie sheet with parchment paper and set aside.
On a clean, lightly floured surface using a lightly floured rolling pin, roll each ball of galette dough into approximately a 6" circle. Transfer to prepared cookie sheet and spread 2 tsp Nutella in center of crust up to 1/2" from edges.
Using a slotted spoon, pile 1/3-1/2 cup apples onto center of crust.
Gently fold over edges to form containing crust (about 1-inch).
Repeat with remaining crust and apples.
Gently brush crusts with the excess caramelized mixture that the apples were cooked in.
Sprinkle crust and filling generously with turbinado sugar.
Bake on 400F for 25-30 minutes.
Allow to cool slightly before serving. Serve with vanilla ice cream drizzled with caramel sauce, if desired.