Make the Choux Pastry:

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

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

3. Once boiling, reduce heat to low and add flour all at once. Stir vigorously with a wooden spoon until a dough ball forms and pulls away from the sides, about 1–2 minutes.

4. Transfer the dough to a mixing bowl. Let cool for 5 minutes.

5. Beat in eggs one at a time, fully incorporating each before adding the next. The final dough should be smooth, glossy, and pipeable.

6. Transfer dough to a piping bag fitted with a round tip. Pipe 1.5-inch rounds onto the prepared baking sheet, spacing 2 inches apart.

7. Wet your fingertip with water and smooth down any peaks.

8. Bake for 25–30 minutes, or until puffed and golden brown. Do not open the oven during baking.

9. Once baked, turn off the oven and crack the door slightly. Let puffs cool in the oven for 10 minutes to prevent collapse. Remove and cool completely on a wire rack.

Make the Lemon Buttercream:
10. In a mixing bowl, beat butter until light and fluffy, about 2–3 minutes.
11. Add powdered sugar gradually and mix until smooth.
12. Add lemon juice, zest, vanilla, and salt. Beat for another 2 minutes until light and spreadable.
13. Transfer filling to a piping bag fitted with a small tip.

Assemble the Lemon Butter Melts:
14. Using a small knife or piping tip, poke a hole in the bottom of each puff.
15. Pipe lemon buttercream into each cream puff until filled.
16. Optional: Dust with powdered sugar or top with a lemon glaze before serving.