✓ Preheat oven to 200°C / 390°F and keep the door closed

✓ Peel, core, and slice apples ¼‑inch thick and toss with 150 g sugar

✓ Combine milk, 50 g sugar, salt, and vanilla paste in a saucepan and heat until just simmering

✓ Whisk egg yolks and stir in 1 tbsp cornstarch until smooth

✓ Temper the yolk mixture with hot milk while whisking

✓ Return mixture to saucepan and cook, stirring, until thick enough to coat the back of a spoon

✓ Stir in the remaining 2 tbsp cornstarch, remove from heat and cover with plastic wrap

✓ Roll out puff pastry sheets to 2 mm thickness, cut into 10 cm squares and place on a parchment‑lined tray

✓ Brush each pastry square with melted butter and sprinkle lightly with sugar

✓ Bake pastry squares 12‑15 minutes until deep golden brown and crisp

✓ Melt butter in a skillet, add apple slices and remaining caramelizing sugar, and cook until edges caramelize

✓ Remove apples from heat and let cool slightly

✓ Layer a spoonful of custard onto the center of each baked puff square

✓ Place 2‑3 caramelized apple slices atop the custard

✓ Cover with a second puff square and press gently to seal edges

✓ Optional: Sprinkle extra sugar and broil 1‑2 minutes until caramelized

✓ Rest assembled mille feuillete 5 minutes before serving

✓ Serve warm, optionally dusted with powdered sugar