Make the Cream Puffs:

1. Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a medium saucepan, combine water, butter, and salt. Bring to a boil over medium heat.
3. Reduce the heat to low and add the flour all at once. Stir vigorously until the mixture forms a ball and pulls away from the sides of the pan.
4. Transfer the dough to a mixing bowl and let it cool for 5 minutes.
5. Add the eggs one at a time, beating well after each addition until the dough is smooth and glossy.
6. Using a piping bag or spoon, drop mounds of dough onto the prepared baking sheet, spacing them about 2 inches apart.
7. Bake for 25-30 minutes, or until golden brown and puffed. Let cool completely on a wire rack.

2. Make the Custard Filling:

1. In a medium saucepan, whisk together milk, sugar, and cornstarch. Heat over medium heat, stirring constantly, until it begins to thicken.
2. In a separate bowl, whisk the egg yolks. Gradually add a small amount of the hot milk mixture to the yolks to temper them, then pour the yolk mixture back into the saucepan.
3. Continue to cook while stirring until the custard is thick and coats the back of a spoon.
4. Remove from heat and stir in vanilla extract and butter. Let cool completely, then chill in the refrigerator.

3. Assemble the Cream Puffs:

1. Slice each cream puff in half and fill with chilled custard using a piping bag or spoon.
2. Replace the tops of the cream puffs and dust generously with powdered sugar.