Preheat oven to 425°. Lightly grease baking sheets or line with parchment paper. Measure flour and set aside.

Bring butter, sugar, salt, and ¾ cup water to a boil in a 3-qt. saucepan over medium-high heat, stirring occasionally. Immediately remove from heat, and quickly stir in flour mixture all at once. 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 ⅝-inch plain tip. Pipe dough onto prepared pans into 1 ½-inch rounds (1 ½ 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 mixture.

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