Preheat your oven to 400°F (200°C).

Line two baking sheets with parchment paper.

Unfold the thawed puff pastry sheet on a lightly floured surface and cut it into three equal strips along the fold marks.

Poke holes in each strip of pastry with a fork.

Place the strips on the prepared baking sheets and bake for about 15 minutes or until golden brown. Allow them to cool on a rack.

In a medium saucepan, bring the milk to a simmer.

In a bowl, whisk together sugar, cornstarch, and salt.

Gradually mix the cornstarch mixture into the simmering milk while stirring continuously, cooking for about 6 minutes until thickened.

Stir in the beaten egg yolks and cook for an additional 2 minutes.

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

Allow the custard mixture to cool to room temperature, then cover and chill in the refrigerator until ready to use.

Once the custard has cooled, carefully cut each layer of pastry into two thinner layers.

Start assembling by spreading a layer of custard on one layer of pastry, topping with another layer of pastry, and repeating the process.

Finish with a layer of dulce de leche on top.

Slice and serve the Dulce de Leche Millefeuille to family and friends.