1️⃣ Prep & preheat:
Preheat oven to 425 °F (220 °C). Line two baking sheets with parchment paper. Fit a pastry bag with a large plain or star tip.

2️⃣ Make the choux pastry:
In a medium saucepan over medium heat, combine water, milk, butter, sugar, and salt. Bring just to a boil, then remove from heat. Add all the flour at once, stirring vigorously with a wooden spoon until the mixture forms a smooth ball and pulls away from the pan.

3️⃣ Incorporate the eggs:
Transfer the dough to a mixing bowl. Beat in eggs one at a time (by hand or with a mixer), fully incorporating each before adding the next. The dough should be glossy and just stiff enough to hold its shape.

4️⃣ Pipe & bake puffs:
Pipe 1½″–2″ mounds onto prepared sheets, spacing 2″ apart. Smooth any peaks with a damp finger. Bake 10 min at 425 °F, then reduce oven temp to 350 °F (175 °C) and bake 15 min more, until puffed and deeply golden. Do not open the oven door during baking. Remove to a wire rack and cool completely.

5️⃣ Make the pastry cream:
In a saucepan, heat milk until steaming. Meanwhile, in a bowl whisk sugar, cornstarch, salt, and yolks until pale. Gradually whisk hot milk into the yolk mixture to temper, then return all to the pan. Cook over medium heat, whisking constantly, until thick and bubbling. Remove from heat; whisk in butter and vanilla. Transfer to a clean bowl, cover surface with plastic wrap, and chill until set, at least 2 hours.

6️⃣ Fill the puffs:
Once pastry and cream are cool, fit a clean pastry bag with a small plain tip. Transfer pastry cream into the bag. Poke a hole in the bottom of each puff and pipe in cream until lightly firm. Alternatively, slice puffs in half and spoon or pipe cream into the cavity.

7️⃣ Garnish & serve:
Dust generously with powdered sugar. For extra flair, drizzle with melted chocolate or warm caramel. Serve immediately.