Preheat your oven to 400°F (200°C) and line two baking trays with parchment paper. Roll out the thawed puff pastry sheets on a floured surface until they are about 1/8 inch thick. Prick the pastry all over with a fork to prevent bubbling.

Place the pricked puff pastry on the prepared trays and bake in the preheated oven for 35-45 minutes until golden brown. Let it cool completely on a wire rack.

Once cooled, carefully trim the baked pastry edges to fit an 8x8 inch pan using a sharp knife.

In a medium saucepan, whisk together granulated sugar, cornstarch, and a pinch of kosher salt to combine thoroughly.

In a separate saucepan, heat whole milk over medium heat until steaming but not boiling.

In a bowl, whisk egg yolks until smooth. Gradually pour in the warmed milk while continuously whisking to temper the yolks.

Return the tempered mixture to the saucepan and cook over medium heat, stirring constantly, until the custard thickens and begins to bubble, about 5-7 minutes.

In your trimmed pan, layer one sheet of puff pastry at the bottom. Pour half of the creamy custard over it, followed by a second layer of pastry and the remaining custard. Chill in the refrigerator for at least 4 hours.

Once set, remove the vanilla slice from the refrigerator and slice using a sharp serrated knife. Dust with powdered sugar before serving.