Thaw the puff pastry according to package instructions. Preheat your oven to 400°F (200°C). Roll out the pastry on a lightly floured surface to about 1/8 inch thick, then cut into rectangles or squares.

In a saucepan, heat the milk over medium heat until it begins to steam.

In a separate bowl, whisk the egg yolks, sugar, and cornstarch until smooth.

Gradually add the heated milk to the yolk mixture, whisking constantly.

Return the mixture to the saucepan and cook over medium-low heat, stirring continuously, until it thickens (about 5-7 minutes).

Remove from heat and stir in the vanilla extract and butter until smooth.

Place the cut pastry pieces on a baking sheet lined with parchment paper.

Spoon the custard onto the center of each piece, then top with blueberries.

Bake for 15-20 minutes, or until the pastries are golden brown and puffed.

Allow them to cool slightly before glazing.

In a bowl, mix powdered sugar with a splash of milk and vanilla extract until smooth.

Drizzle over the cooled pastries.

Enjoy warm or at room temperature, garnished with additional blueberries if desired.