Step 1: Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Step 2: In a medium saucepan, combine water, milk, butter, sugar, and salt. Bring to a boil over medium heat.
Step 3: Remove from heat and stir in the flour all at once with a wooden spoon until a smooth dough forms.
Step 4: Return the pan to low heat and cook for 2 minutes, stirring constantly, until a film forms on the bottom of the pan.
Step 5: Transfer the dough to a bowl and let cool for 5 minutes. Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next.
Step 6: Pipe 2-inch mounds of dough onto the baking sheet and bake for 20 minutes, then reduce heat to 350°F (175°C) and bake for another 15 minutes.
Step 7: Poke a small hole in the side of each puff to release steam and let cool completely on a wire rack.
Step 8: Whisk together heavy cream, sugar, and matcha powder until stiff peaks form to create the matcha filling.
Step 9: Pipe the matcha cream into the cooled shells and serve immediately.