Classic choux pastry shells filled with crème légère: a lightened vanilla pastry cream. The consistency tests built into this recipe take the guesswork out of choux so your shells come out hollow and crisp every time.
Make the pastry cream. Warm the milk in a saucepan. Whisk yolks, sugar, and cornstarch together in a bowl until pale. Temper the hot milk into the yolk mixture, return to the pan, and cook over medium heat, whisking constantly, until the mixture comes to a boil and thickens. Continue cooking at a gentle bubble for 1 to 2 minutes to cook out the starch. Remove from heat, whisk in butter and vanilla, and press plastic wrap directly onto the surface. Refrigerate until fully chilled.
Make the choux paste (panade). Combine water, milk, butter, sugar, and salt in a saucepan over medium heat. Bring to a full boil. Add the flour all at once and stir vigorously with a wooden spoon until the dough pulls cleanly from the sides of the pan and a thin film forms on the bottom, about 1 to 2 minutes. The paste should look smooth and matte.
Add the eggs. Transfer the panade to a stand mixer fitted with the paddle attachment and mix on medium speed for 1 to 2 minutes to release steam. Add eggs one at a time, allowing each to fully incorporate before adding the next. Stop adding eggs when the paste holds a trench when a spoon is dragged through it, falls from a spoon in a thick ribbon, and hangs in a V-shaped hook. Do not add eggs beyond this stage.
Pipe and egg wash. Transfer choux paste to a piping bag fitted with a round tip. Pipe rounds onto parchment-lined baking sheets. Whisk together the egg wash and lightly brush each round, smoothing any peaks with a dampened finger.
Bake. Bake at 375°F conventional (or 350°F convection) until the shells are deep golden brown all over, including the creases, 30 to 35 minutes. Do not open the oven during the first 25 minutes. Cool completely on a wire rack before filling.
Make the crème légère. Whisk the chilled pastry cream until smooth. Whip the heavy cream to medium-stiff peaks separately, then fold it into the pastry cream in two additions until fully combined and light.
Fill and serve. Pipe crème légère into the bottom of each shell or slice the tops and fill. Serve immediately for the best texture.
Notes
Storage: Store choux paste in a piping bag in the refrigerator for up to 3 days. Store piped, egg-washed unbaked rounds tightly wrapped in the refrigerator for up to 3 days or frozen for up to 1 month; bake from the refrigerator or allow frozen rounds to soften in the fridge for 1 hour before baking. Unfilled baked shells freeze for up to 1 month; refresh in a 350°F oven until crisp, cool completely, then fill. Filled cream puffs can be refrigerated for up to 3 days, but the texture is best within a few hours of filling. Doneness cue: The shells must be deep golden brown all over, including all creases. A pale shell will collapse as it cools. If uncertain, bake an additional 3 to 5 minutes. Egg quantity: The number of eggs will vary based on egg size, humidity, and how long the panade cooked. Trust the consistency tests over the recipe quantity: trench, ribbon, and hook before you stop.