Preheat the oven to 400°F. Line 2 baking sheets with parchment paper.
In a medium saucepan, bring butter, milk, water and salt to a boil over medium heat. Remove from heat and stir in flour.
Return to heat and cook, stirring constantly with a wooden spoon, until mixture starts to pull away from the sides of the pan, about 3~4 minutes.
Transfer the mixture to the bowl of a stand mixture or to a mixing bowl. Using a paddle attachment or a hand mixer, mix until the mixture is cool slightly, about 1 minute. With the machine running, add eggs, one at a time, mixing well after each addition.
Transfer dough to a piping bag fitted with a ½” round or star tip.
Pipe 20 2½” long eclairs onto the baking sheet.
Bake until the eclairs rise and are golden brown, about 20~25 minutes.
Transfer to wire rack to cool.