Preheat oven to 375°F (190°C). Using a permanent marker and a 2-inch round cutter as a guide, draw 30 circles onto 2 sheets of parchment paper. Turn parchment over, and place on 2 baking sheets.

In a medium saucepan, melt butter over medium heat. Add ½ cup (120 grams) water, milk, granulated sugar, and salt; bring to a boil. Using a wooden spoon, stir in flour. Cook, stirring constantly, until a skin forms on bottom of pan, 1 to 2 minutes. Transfer mixture to the bowl of a stand mixer fitted with the paddle attachment. With mixer on low speed, beat until dough is warm to the touch, about 1 minute. Add eggs and egg white, one at a time, beating until combined.

Transfer batter to an 18-inch piping bag fitted with a ½-inch round tip*. Place piping tip in center of one drawn circle. Hold tip perpendicular ½ inch above parchment paper. Holding tip stationary the entire time, apply even pressure until batter reaches edges of drawn circle. Stop applying pressure, and lift up. Wet your finger with water, and press down tip to create a smooth top. Repeat with remaining batter.

Bake for 15 minutes. Rotate pans, and bake until fully puffed and deep golden brown, about 10 minutes more. Let cool completely on wire racks.

Using a ¼-inch round piping tip, poke a hole in center of bottom of each puff. Insert a skewer into hole, and move it around to ensure cream will fill entire puff. Fill a pastry bag fitted with same tip with Vanilla Crème Mousseline. Place piping tip into hole, and apply gentle pressure to fill. Repeat with remaining puffs and Vanilla Crème Mousseline. Dust with confectioners’ sugar, if desired. Serve immediately, or refrigerate until ready to serve. Best served same day.