Preheat oven to 400°F.
Pulse flour and sugar in a food processor. Add diced butter and pulse for about 20-30 seconds, until the butter is broken up. Add the vinegar and run until dough comes together in the machine.
Turn dough out into a 10” tart pan with a removable bottom. Press with your hands up the sides and across the bottom. It will seem there isn’t enough dough - but there is, just keep working it!
Place a sheet of parchment paper over the bottom and up the sides of the tart. Fill the tart shell with pie weights (or beans). Bake for 15 minutes. Remove the tart shell from the oven and carefully remove the parchment and weights; poke the bottom of the tart with the tines of a fork. Return to the oven to bake until light golden and cooked through, approximately 10-15 more minutes. Cool before filling.
When ready to fill the tart shell, make the filling. Whisk the cornstarch into ¼ cup milk in a small bowl. Add the remaining 1½ cups milk, Nutella, salt, and extracts to a pan. Whisk over medium heat until the Nutella melts, then whisk in the cornstarch mixture.
Bring mixture to a boil, stirring constantly with a rubber spatula or wooden spoon. It will start to thicken. Once the mixture starts to boil, stir constantly until fully thickened, about 2 minutes (it will coat the back of the spoon). Pour into the crust.
Cool 15 minutes, then cover the top with plastic wrap to prevent a skin from forming and chill the tart until cold and set, at least 3 hours.
Top with strawberries before serving, if desired. I also melted about 2 tablespoons Nutella in the microwave (about 30 seconds) and drizzled it over the berries for a pretty topping.