Step 1

Line 2 baking sheets with parchment paper. Fit 1 large pastry bag (or a plastic freezer bag with 1/2" cut from one bottom corner) with plain 1/2" tip.

Step 2

Bring milk, butter, 1 teaspoon sugar, salt, and 1/2 cup water to a boil in a medium saucepan over medium heat, stirring occasionally. Add flour all at once; reduce heat to medium-low. Stir vigorously with a wooden spoon until a dough forms and pulls away from sides of pan, 1-2 minutes. Continue beating vigorously until a thin dry film forms on bottom and sides of pot, about 1 minute. Transfer to a bowl.

Step 3

Add 1 egg and stir vigorously with wooden spoon until egg is incorporated and dough looks dry again, about 2 minutes. Repeat with 4 more eggs, adding one at a time and stirring vigorously to incorporate before adding the next. Dough should be smooth, shiny, and thickened.

Step 4

Spoon dough into prepared pastry bag; pipe out 2 1/2"-diameter rounds on prepared sheets, leaving 2" between rounds. DO AHEAD: Freeze the piped dough rounds on the baking sheets, then transfer to resealable plastic bags and freeze for up to 1 month. Place on parchment paper-lined baking sheets, spacing apart (do not defrost) before continuing with recipe.

Step 5

Arrange racks in upper and middle thirds of oven and preheat to 450°F. Whisk remaining egg with 2 teaspoons water and brush dough rounds all over with egg wash.

Step 6

Transfer baking sheets to oven; turn oven off. After 10 minutes, heat oven to 350°F and bake for 10 minutes. Rotate pans front to back and top to bottom; continue baking until deep golden brown all over, about 10 minutes longer (puffs will deflate if removed from oven before fully baked). Transfer puffs to a wire rack and let cool completely.

Step 7

Using a serrated knife, gently slice the top quarter off each puff; transfer tops to a plate. With your finger, gently push down the soft film of cooked dough inside each puff.

Step 8

Prepare a second pastry bag (or freezer bag) with 1/2" open-star tip. Beat heavy cream and remaining 2 tablespoons sugar in a large bowl until soft peaks form. Spoon whipped cream into prepared pastry bag. Fill each puff with cream, finishing with a generous ring of cream on top. Dust tops of cream puffs with powdered sugar; place atop puffs.