Preheat your oven to 400°F (200°C). Line two baking sheets with parchment paper.

In a medium saucepan, bring 1 cup of water and ½ cup of unsalted butter to a rolling boil. Add 1 cup of all-purpose flour and stir until a ball forms.

Remove from heat and let cool for 5 minutes. Beat in 4 room-temperature eggs one at a time until smooth.

Transfer choux pastry to a piping bag. Pipe two rectangles onto the prepared baking sheets, about 10x6 inches.

Bake for 25-30 minutes until puffed and golden. Cool on a wire rack.

In a saucepan, combine 2 ½ cups of whole milk, ½ cup of sugar, and ¼ cup of cornstarch. Cook and whisk until thick, about 6-8 minutes.

Remove from heat, whisk in 4 beaten egg yolks and 1 tablespoon of vanilla extract.

On a serving platter, place one pastry layer, spread with vanilla custard, and top with the second layer.

Cover lightly with plastic wrap and chill for at least 2 hours before serving.