Melt 2 tablespoons butter in a large skillet over medium heat. Add the diced apples, brown sugar, 1 teaspoon cinnamon, optional nutmeg, and lemon juice, then cook for 6–8 minutes, stirring occasionally, until tender but not mushy.

Stir the cornstarch and water together in a small bowl to make a slurry. Add it to the skillet and cook for 1–2 minutes, stirring, until the apple juices thicken; stir in vanilla if using, then cool the filling slightly.

Heat the oven to 375°F (190°C). Brush both sides of each flour tortilla lightly with the 4 tablespoons melted butter, then coat both sides in the mixture of granulated sugar and 1 1/2 teaspoons cinnamon.

Fold each tortilla into a taco shape and place it over the bars of an oven rack, or support it in an inverted muffin tin. Bake for 8–12 minutes, until crisp and lightly browned, then cool for several minutes so the shells firm.

Spoon the warm apple filling into the crisp shells. Serve immediately with vanilla ice cream, whipped cream, and optional caramel sauce.