Move oven rack to the middle position. Preheat oven to 375 ° F.
Line a rimmed baking sheet with parchment paper or a silicone baking mat.
MAKE CREAM PUFF SHELLS:
Make an egg wash with 1 egg and 1 tablespoon of water, whisk together vigorously, and set aside.
In a large saucepan, add water, butter, salt, and sugar.
Bring the mixture to a boil over medium-high heat and then remove from the saucepan from the heat.
Quickly stir in flour with a wooden spoon until well combined and thick paste forms, about 1 minute.
Return the saucepan to the stovetop and continue cooking, constantly stirring until a film forms on the bottom of the saucepan.
Remove the saucepan from the heat, transfer the mixture to a large bowl, and let it cool slightly.
Add 4 eggs, one at a time, stirring to incorporate each egg completely before adding the next egg.
Transfer dough mixture to a large disposable pastry bag fitted with a large round tip.
Pipe circles, 2 inches in diameter, onto the prepared baking sheets.
Moisten your finger with water and gently smooth out the pointed peaks of the dough if necessary.
Brush the surface of each cream puff with egg wash.
Bake the cream puff shells at 375 ° F for 25-30 minutes or until the puffs rise and are golden brown.
Remove the pan from the oven and allow the cream puff shells to cool completely on wire racks.
MAKE CREAM PUFF FILLING:
While cream puffs are cooling, place a large mixing bowl and beaters in the freezer to chill.
When the cream puffs are cooled, remove the bowl and beaters from the freezer.
Add heavy cream to the chilled, large bowl and beat until peaks form.
Add powdered sugar and vanilla and continue beating until stiff peaks form.
Fit a large pastry bag with a round or decorative tip and fill it with the cream puff filling.
ASSEMBLE CREAM PUFFS:
Cut the top portion of each cream puff off with a knife.
Then, fill each shell with pastry cream.
Next, replace the tops of the cream puffs.
Alternatively, you can leave the cream puffs whole and fill them from the puff's bottom using a nozzle tip.
Dust the puffs with powdered sugar or berries, if desired.
Add the cream to the puffs no more than 4 hours before serving. Although the puffs will keep for 24 hours in the refrigerator, their filling will deflate slightly, so it's best to fill them no more than 4 hours before serving.