Preheat oven to 400°F (200°C).

Roll out puff pastry sheets on a lightly floured surface and trim to fit a 9x9-inch baking pan.

Place pastry sheets on parchment-lined baking trays. Prick with a fork to prevent excessive puffing.

Bake 10–12 minutes until golden and crisp. Cool completely.

In a saucepan, heat milk and cream over medium heat until just simmering.

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

Slowly pour half of the hot milk mixture into the egg mixture while whisking constantly to temper.

Return mixture to saucepan and cook over medium heat, whisking continuously until thick and smooth, about 3–5 minutes.

Remove from heat and stir in vanilla extract and butter until creamy.

Cover with plastic wrap directly on the custard surface and cool completely.

Line a 9x9-inch pan with one cooled pastry sheet.

Spread custard evenly over the pastry layer.

Top with second pastry sheet.

Refrigerate at least 2 hours until firm.

Dust generously with powdered sugar before slicing into neat Vanilla Custard Cream Squares. Serve chilled.