Preheat your oven to 425°F and lightly grease 9 muffin cups in a standard 12-cup muffin tin.

In a medium skillet over medium-high heat, add the diced apples, butter, brown sugar, and cinnamon. Stir and cook for 2–3 minutes until the apples begin to soften and release liquid.

Lower the heat to medium, cover the pan, and cook for 10 minutes. Then uncover and cook 2–3 more minutes, stirring occasionally, until the mixture is thick and most of the moisture has evaporated.

On a lightly floured surface, unfold and gently roll out the thawed puff pastry into an approximately 11×11-inch square. Cut into 9 equal squares.

Place one pastry square into each greased muffin cup, gently pressing the center down and letting the corners hang over the edges.

Spoon about 2 1/2 to 3 tablespoons of the apple filling into each cup, filling them about 3/4 of the way.

Fold the four corners of the dough over the filling, pinching gently in the center. Leave small gaps for air to escape during baking.

If using, brush the tops with egg wash for a richer golden color.

Bake on the center rack for 20–25 minutes or until puffed and golden brown.

Let cool in the pan for about 8–10 minutes, then transfer to a wire rack.

Dust with powdered sugar before serving, using a sieve or your fingers.