1. Preheat the oven to 400 degrees. Prepare a pastry bag with a 1/2-inch tip (optional).

2. In a small bowl, whisk together the flour and salt. Place the water and butter in a medium saucepan and bring to a boil. Add the flour mixture and beat vigorously with a wooden spoon for several seconds, until the dough is smooth, pulls away from the sides of the pan, and begins to form a ball. Remove the pan from the heat. Cool the dough for a couple of minutes. Beat in the eggs one at a time, until smooth and glossy, 1 to 2 minutes.

3. Place the dough in the pastry bag. (Alternatively, you can use a spoon to form the puffs.) Grease 2 rimmed baking sheets or line with baking parchment. Pipe the desired shapes onto the baking sheets. The dough can be frozen at this point on the tray then collected into freezer bags and sealed.

4. For small (1 1/2-inch) puffs, bake for 10 to 12 minutes. For a standard eclair shape, bake for 10 minutes then raise the heat to 425 degrees and continue baking for 15 minutes total, or until puffed up, golden brown, and firm to the touch. Cool completely on a wire rack.