Preheat oven to 425°.

Line baking sheets with parchment paper. Prepare cream puff ingredients.

Bring butter, sugar, salt, and 3/4 cup water to a boil in a 3-qt. saucepan over medium-high heat, stirring occasionally. Remove from heat and immediately stir in flour. Return to heat and stir with a wooden spoon for 1 to 2 minutes, or until dough is smooth and forms a ball. Transfer dough to the bowl of an electric mixer, and let cool 5 minutes.

Add 3 eggs, 1 at a time, beating until mixture is smooth and glossy. Spoon dough into a pastry bag fitted with a 5/8-inch plain tip. Pipe dough onto prepared pans into 1 1/2-inch rounds (1 1/2 inches high). Smooth out peaks and round tops with a moistened finger.

Whisk together remaining 1 egg and 1 tsp. water. Brush tops of dough with egg wash.

Bake at 425°F for 5 minutes; reduce oven temperature to 375°F, and bake 30 minutes or until puffy and golden brown. Turn oven off; allow puffs to stand in closed oven 10 minutes. Remove from baking sheets to wire racks, and cool completely before filling.

Using stand mixer fitted with whisk attachment; beat heavy cream and salt on medium/high until soft peaks form. Add vanilla and sugar. Beat until stiff peaks form. Transfer to piping bag fitted with small tip. Insert tip into puffs and gently fill. Dust puffs with powdered sugar.