While the dough is rising, prepare the apple filling by combining the diced apples, brown sugar, butter, lemon juice, vanilla, cinnamon and cornstarch in a large skillet and cooking over medium heat, stirring occasionally, until the apples soften and release their juices. The juices will thicken slightly from the cornstarch, but it will still be very syrupy.
3-4 apples, 1 cup (200g) brown sugar, 2 Tablespoons salted butter, 1 Tablespoon lemon juice, 1 teaspoon pure vanilla extract, 1 teaspoon ground cinnamon, 2 Tablespoons cornstarch