Position a rack in the middle of the oven and heat the oven to 400°F. Cut a sheet of parchment to fit in a heavy-duty 13x18-inch rimmed baking sheet. Using a pencil, draw three sets of two lines spaced 3 inches apart, running the length of the parchment. These will be guidelines for piping the éclair dough. Line the baking sheet with the parchment, penciled side down—you should be able to see the lines through the parchment. If not, draw them darker.
In a medium saucepan, bring the butter, salt, and 1 cup water to a boil over medium-high heat. Reduce the heat to medium and add the flour. Using a wooden spoon, stir vigorously to combine. Continue to stir, using a figure-eight motion and smearing the dough against the sides of the pan to cook the flour and work out any lumps, for 2 minutes. The mixture will be thick and look like a firm ball, or balls, of sticky mashed potatoes that pull away from the pan sides. During this process, it’s normal for a thin layer of dough to stick to the bottom of the pan and sizzle.
Stir the dough.
Remove the pan from the heat and scrape the dough into the bowl of a stand mixer fitted with the paddle attachment. On low speed, mix until the dough feels merely warm to the touch, not hot, 3 to 5 minutes.
With the mixer still on low, beat in the eggs one at a time. After each egg is added, the dough will separate into small lumps and then come back together. After the dough pulls back together, briefly (about 20 seconds) increase the speed to medium low to mix the dough well. Reduce the speed to low before adding the next egg. After the addition of the last egg, scrape the bowl well and beat on medium low for a final 30 seconds.
Scrape the dough into a pastry bag fitted with a large star tip. Twist the top of the bag to push the dough toward the tip. Hold the bag at a 60-degree angle and set the tip of the pastry tube on the paper, right at the top of one of the 3-inch-wide stripes you drew earlier. Squeeze the pastry bag and, using the lines on the parchment as a guide, pipe out 3-inch lengths of dough in a tight zigzag pattern, spacing the éclairs about 1 inch apart.
Bake until the shells are puffed, crisp, and thoroughly golden brown, 45 to 50 minutes. If you find that they’re baking unevenly, rotate the pan. Remove from the oven, transfer to a rack, and let cool completely, about 15 minutes, before filling or storing.
Store the shells in an airtight container at room temperature for up to two days. Or freeeze them in an airtight plastic freezer bag for up to six weeks. To refresh the shells before assembling. arrange them on a baking sheet and warm them in a 350°F oven until dry, firm, and almost crisp, 10 to 15 minutes for room temperature shells, 15 to 20 for frozen shells. Let cool before using.