Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a saucepan, combine water, butter, and salt. Bring to a boil over medium heat.

Remove from heat and add flour all at once. Stir vigorously until a smooth dough forms.

Return to low heat and cook, stirring constantly, for 1-2 minutes to dry out the dough.

Transfer the dough to a mixing bowl and let cool slightly. Add eggs one at a time, mixing well after each addition. The dough should be smooth and pipeable.

Transfer dough to a piping bag fitted with a large round tip. Pipe 1-inch rounds onto the prepared baking sheet.

Bake for 25-30 minutes, or until golden brown and puffed. Let cool completely on a wire rack.

In a saucepan, whisk together milk, sugar, cornstarch, and salt.

Whisk in egg yolks. Cook over medium heat, stirring constantly, until the mixture thickens and comes to a boil.

Remove from heat and stir in vanilla extract. Transfer to a bowl, cover with plastic wrap (pressing the wrap directly onto the surface to prevent a skin from forming), and refrigerate until completely chilled.

Once the cream puffs and pastry cream are completely cooled, cut the cream puffs in half horizontally.

Fill each cream puff with pastry cream. Dust with powdered sugar before serving.