Preheat your oven to 375°F (190°C). On a clean, floured surface, roll out thawed puff pastry and cut it into 6 equal squares. Score a smaller square within each without cutting through, creating a border. Brush the edges with beaten egg and sprinkle granulated sugar over the top. Bake for 15–18 minutes until they puff up and turn golden brown. Once done, let them cool, then gently press the centers to create wells.

In a medium saucepan, heat 2 cups of whole milk over medium heat until it's steaming but not boiling. While the milk heats, whisk together 4 egg yolks, 1/2 cup of granulated sugar, 2 tablespoons of cornstarch, and a pinch of salt in a separate bowl until smooth. Gradually pour the hot milk into this mixture, whisking continuously. Return everything to the saucepan and cook on medium-low heat for 5–7 minutes, stirring until thickened. Remove from heat, stir in 1 teaspoon of vanilla extract, and let it cool.

Once the pastry shells have completely cooled and the custard is chilled, fill each pastry well with the rich vanilla custard until just overflowing. Carefully top with a vibrant selection of fresh berries, such as raspberries, blueberries, and strawberries, allowing their natural sweetness to complement the custard. For an extra touch, dust the pastries lightly with powdered sugar and garnish with fresh mint leaves if desired.