1. Prepare the Crust:

1. Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a small bowl, combine mixed berries, 1/2 cup sugar, cornstarch, and lemon juice. Set aside.
3. In a saucepan, whisk together milk, egg yolks, 1/4 cup sugar, and flour. Cook over medium heat, stirring constantly, until thickened. Remove from heat and stir in vanilla extract. Let cool slightly.
4. Roll out puff pastry on a floured surface to a 12×10-inch rectangle. Spread the custard evenly over the pastry, leaving a 1-inch border on all sides.
5. Spoon the berry mixture over the custard. Carefully roll up the pastry from the long side, tucking in the ends. Place seam-side down on the prepared baking sheet.
6. Brush the top with melted butter and make a few slits to vent steam. Bake for 25-30 minutes until golden brown. Let cool, then dust with powdered sugar before serving.