Choux Pastry Layer

Preheat the oven to 400°F (200°C) and line two baking trays with parchment paper. Set them aside.

In a medium saucepan over medium heat, combine the water and butter and bring the mixture to a boil. Once boiling, add the flour and stir vigorously with a wooden spoon until the dough pulls away from the sides of the pan and a thin starchy film forms at the bottom—this should take about 2 minutes.

Remove the pan from the heat and transfer the dough to a stand mixer fitted with a paddle attachment. Mix for about 2 minutes to allow the dough to cool slightly.

Add the eggs one at a time, ensuring each egg is fully incorporated before adding the next.

Once all the eggs are added, give the dough a thorough final mix. Divide the dough into two equal portions and spread each portion onto the prepared baking sheets using an offset spatula to form rectangles. Aim to make both layers roughly the same size, though they don't need to be perfect.

Bake for 25 to 30 minutes or until golden brown and puffed. Remove from the oven and let the pastry layers cool completely on the baking trays.

Vanilla Custard Filling

In a bowl, whisk together the eggs, vanilla extract, cornstarch, and 1 cup of milk until smooth. Set aside.

In a medium saucepan, combine the remaining 2 cups of milk with the sugar and butter. Bring to a boil over medium heat.

Slowly pour in the egg mixture, whisking constantly. Continue to whisk for 2 to 3 minutes, or until the mixture thickens and starts to bubble. Remove from the heat immediately.

Spread the hot custard evenly over one of the baked choux pastry layers using an offset spatula, leaving about a 1-inch border around the edges.

Carefully place the second choux pastry layer on top and gently press down so it adheres to the custard.

Refrigerate the assembled pastry for at least 1 hour, or until the custard is fully set and cooled. Dust with powdered sugar before slicing into squares.

Store any leftovers covered in the refrigerator for up to 3 days.