In a medium saucepan, combine 1 cup of water, ½ cup of butter, and a pinch of salt. Bring to a rolling boil over medium heat. Stir in 1 cup of all-purpose flour until the dough pulls away from the sides.

Remove from heat, let cool slightly, then add 4 large eggs one at a time, mixing vigorously after each addition until smooth and glossy.

Preheat oven to 400°F (200°C). Line two baking trays with parchment paper. Divide dough evenly, spreading to form two layers. Bake for 30-35 minutes until puffed and golden brown.

Heat 2 cups of milk over medium heat until hot but not boiling. In a bowl, whisk together ½ cup of sugar, 4 large egg yolks, and ¼ cup of cornstarch. Gradually mix in some hot milk, then return the mixture to the saucepan and cook until thickened.

Remove custard from heat, stir in 1 tablespoon of vanilla extract. Allow both pastries and custard to cool completely. Spread custard over one layer of pastry, placing the second layer on top.

Cover with plastic wrap and refrigerate for at least 2 hours before serving. Dust with powdered sugar if desired.