Preheat your oven to 400°F (200°C). In a saucepan, bring 1 cup of water, 1/2 cup of butter, and a pinch of salt to a rolling boil. Stir in 1 cup of all-purpose flour until it forms a ball.

Once cooled slightly, add 4 large eggs one at a time, beating well after each addition until the dough is smooth and glossy.

Spread the choux mixture into two rectangles on a baking sheet lined with parchment paper. Bake for 25-30 minutes until golden brown.

In a medium saucepan, combine 2 cups of milk, 1 cup of sugar, 1/3 cup of cornstarch, and 4 beaten eggs. Cook over medium heat, whisking continuously until thickened.

Remove from heat and stir in 2 teaspoons of vanilla extract and 3 tablespoons of butter until smooth. Allow custard to cool.

Place one piece of cooled choux pastry on a serving platter, spread half of the custard filling, and top with the second piece of pastry.

Cover lightly with plastic wrap and refrigerate for at least 3 hours before serving.