1. Prepare the Crust:

1. Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
2. In a saucepan, combine water, butter, and salt. Bring to a boil, then remove from heat and stir in flour until a dough forms. Return to low heat, stirring for 2 minutes to dry slightly.
3. Transfer dough to a bowl. Beat in eggs one at a time until smooth and glossy. Pipe or spoon dough into mounds on the baking sheet. Bake for 20 minutes, then reduce heat to 350°F (175°C) and bake for 10 more minutes until golden. Cool completely.
4. For the cream, heat milk in a saucepan until simmering. Whisk sugar, egg yolks, and cornstarch in a bowl. Gradually whisk hot milk into egg mixture, then return to saucepan. Cook over medium heat, stirring constantly, until thick. Remove from heat, stir in vanilla and butter. Chill until set.
5. Slice cooled choux puffs in half and fill with vanilla cream. Serve immediately or dust with powdered sugar.