1. Make the cream puff shells:

1. Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a medium saucepan, combine water, milk, butter, sugar, and salt. Heat over medium heat until the butter melts and the mixture begins to boil.
3. Reduce heat to low and add the flour all at once. Stir vigorously with a wooden spoon until the dough forms a ball and pulls away from the sides of the pan (about 2-3 minutes).
4. Remove from heat and let the dough cool for 5 minutes.
5. Add the eggs one at a time, mixing well after each addition until the dough is smooth and shiny.
6. Transfer the dough to a piping bag fitted with a round or star tip. Pipe small mounds (about 1 1/2 inches wide) onto the prepared baking sheet, spacing them about 2 inches apart.
7. Bake for 20-25 minutes, or until the puffs are golden brown and puffed up. Avoid opening the oven during baking.
8. Remove from the oven and pierce each puff with a toothpick to release steam. Let them cool completely.

2. Prepare the rhubarb filling:

1. In a medium saucepan, combine the rhubarb, sugar, lemon juice, and water. Cook over medium heat, stirring occasionally, until the rhubarb softens and breaks down into a sauce (about 5-7 minutes).
2. Remove from heat and let the rhubarb mixture cool completely.
3. In a mixing bowl, whip the heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
4. Gently fold the cooled rhubarb mixture into the whipped cream. Refrigerate until ready to use.

3. Assemble the cream puffs:

1. Slice the cream puff shells in half horizontally.
2. Spoon or pipe the rhubarb cream filling into the bottom half of each shell.
3. Place the top half of the shell back on and dust with powdered sugar.