Preheat the oven to 400 °F. Line a ½ sheet pan with parchment paper or a silicone baking mat.
Scoop or pipe 1 ½" mounds of batter onto the sheet pan, spaced about 2" apart. Use a wet fingertip to tamp down any points on top of the batter rounds. You can adjust the size of the cream puffs by scooping larger or smaller mounds of batter.
Bake until well puffed and golden brown, about 20 minutes. Remove the puffs from the oven and turn off the heat. Use a small paring knife to form a small hole in the bottom of each puff. Return the tray to the still warm (turned off) oven and allow the puffs to dry for at least 30 minutes. Cool to room temperature before filling.
Make the filling & Assemble
Whip the cream with the powdered sugar. Fold the cream into the pastry cream in two batches. Don't overstir! Scoop the cream into a pastry bag fitted with a small, plain tip. Pipe the filling into the cream puffs, making sure each puff is completely filled.
Refrigerate until serving. Sprinkle with powdered sugar just before serving.
Notes
The empty puffs can be stored at room temperature for several days or packed into freezer bags and frozen for up to a month. Rewarm in a 200F oven to re-crisp the shells before filling.