Prepare the Custard

1. In a saucepan, warm the milk until hot but not boiling.
2. In a bowl, whisk egg yolks and sugar until smooth, then whisk in cornstarch.
3. Slowly pour in a little hot milk while whisking, then return the mixture to the saucepan.
4. Cook over medium heat, whisking constantly, until thickened and bubbling. Add vanilla and optional butter, then remove from heat.

Prepare the Pastry Shells

1. Preheat oven to 400°F (200°C).
2. Cut puff pastry into 4-inch squares and score a smaller square inside each without cutting through.
3. Brush the edges with egg wash and bake for 15-18 minutes until golden.

Assemble the Pastries

1. Let pastry shells cool on a rack.
2. Fill cooled shells with pastry cream and top generously with fresh berries.