Preheat your oven to 375 degrees F and line a baking sheet with parchment paper.

Make the filling. Melt the butter in a small skillet over medium heat. Add the diced apples, brown sugar, cinnamon, and nutmeg. Stir and cook for 3 to 4 minutes until the apples soften slightly but still hold their shape. Stir together the cornstarch and cold water in a small bowl, then pour it into the apple mixture. Stir for 1 minute until the juices thicken and coat the apples. Remove from heat and stir in the vanilla. Let the filling cool for 5 minutes.

Open the crescent roll cans and unroll the dough. Press every two triangles together along their seam to form a rectangle. Pinch the diagonal seam firmly with your fingers so it stays sealed. You will have 8 rectangles total.

Spoon about 1 1/2 to 2 tablespoons of apple filling onto the lower half of each dough rectangle, keeping a half-inch border around the edges. Do not overfill or the pies will be hard to seal.

Fold the top half of the dough down over the filling to form a smaller rectangle. Press the edges together with your fingers, then crimp all the way around the border with a fork to seal each pie. Press firmly so the filling does not leak during baking.

Place the sealed pies on the lined baking sheet about 1 inch apart. Use a sharp knife or fork to poke two small vent holes in the top of each pie so steam can escape.

Bake for 14 to 16 minutes until the tops are deep golden and the dough feels set when you press it lightly. The undersides should also be golden, not pale.

While the pies bake, whisk together the powdered sugar, milk, vanilla, and salt in a small bowl until smooth. The glaze should be pourable but not watery. Add milk a few drops at a time to reach the right consistency.

Let the pies rest on the pan for 3 minutes after they come out of the oven, then drizzle the glaze generously over the top of each one while they are still warm. Serve warm or at room temperature.