Preheat oven to 400°F (200°C). Roll out gluten-free puff pastry and cut into evenly sized rectangles. Prick with a fork and place on a parchment-lined baking sheet.

1. Place another sheet of parchment and a second baking tray on top to weigh it down. Bake for 15–20 minutes or until golden and crisp. Let cool completely.
2. In a skillet, melt butter over medium heat. Add sliced apples, brown sugar, and cinnamon. Cook until tender and caramelized, about 8–10 minutes. Let cool.
3. In a saucepan, whisk together milk, sugar, cornstarch, salt, and egg yolks. Cook over medium heat, stirring constantly, until thickened (about 5–7 minutes).
4. Remove from heat and stir in vanilla extract. Let pastry cream cool completely, covering surface with plastic wrap to prevent skin from forming.
5. To assemble, place one puff pastry rectangle on a serving plate. Spread with a layer of pastry cream and top with caramelized apples.
6. Repeat layers two more times, ending with a pastry layer on top.
7. Refrigerate assembled mille feuille for at least 1 hour to set.
8. Before serving, dust with powdered sugar or drizzle with glaze if desired. Slice with a serrated knife and serve chilled.