The foundation of the perfect cream puff is light, airy pâte à choux, which is made with eggs, flour, and butter. Once the cream puffs are shaped and baked, they're filled with a rich homemade pastry cream.
Preheat oven to 375 degrees. Line two baking sheets with parchment paper.
In a medium saucepan over medium-high heat, combine butter, sugar, salt and 1 cup water. Bring to a boil and quickly stir in the flour with a wooden spoon. Continue to stir until a film forms on the bottom of the pan.
Remove from heat and transfer contents to a bowl to cool slightly, about 3 minutes. Add 4 eggs, one at a time, stirring vigorously to entirely incorporate egg after each addition.
For the egg wash, whisk together the remaining egg with 1 tablespoon water. Set aside.
Transfer the pate a choux to a large pastry bag fitted with a 5/8-inch plain tip. Pipe 1-1/2-inch rounds onto each prepared pan. Gently smooth the pointed peaks with a moistened finger, rounding tops to ensure even rising. Brush tops with reserved egg wash. Bake until puffs rise and are golden brown, about 30 minutes. Let cool on sheets on wire racks. Puffs can be stored at room temperature for up to 1 day.
Transfer pastry cream to a pastry bag fitted with a 1/8-inch plain round tip. Insert the tip into the opening of each pastry, and pipe to fill with pastry cream. Serve immediately.