Pre-heat the oven to 425°F.
Line two baking sheets with parchment paper.
Bring the milk, water, butter, sugar, and salt to a rapid boil in a medium sturdy saucepan over high heat.
Add the flour all at once and lower the heat to medium-low, and immediately start stirring vigorously with a wooden spoon or heavy whisk.
The dough will come together, and a light crust will form on the bottom of the pan.
Keep stirring for another minute or two to dry out the dough. The dough should be very smooth.
Turn the dough into the bowl of a stand mixer fitted with the paddle attachment, or into a bowl you can use to mix with a hand mixer or a wooden spoon and a good amount of arm muscle.
Let the dough sit for a minute, then add the eggs one by one and beat until the dough is thick and shiny.
Make sure each egg is completely incorporated before you add the next, and don't be concerned if the dough falls apart - by the time the last egg goes in, the dough will come together again.
Fit a large pastry bag with a large (2/3-inch diameter) plain tip.
Fill the pastry bag with half of the dough and pipe out thick strips of dough that are 4 to 4 1/2 inches long on the first baking sheet; keep the strips about 2 inches apart so they'll have room to puff.
Pipe the other half of the dough onto the second baking sheet (the eclairs at this point can be frozen for up to 2 months).
Bake the eclairs for 10 minutes, then reduce the oven temperature to 375°F and continue baking for another 7 minutes, then wedge the handle of wooden spoon into the oven door so it stays slightly ajar, and bake for about 3 minutes more, or until the eclairs are golden, firm, and puffed.
Transfer the eclairs to a rack and cool to room temperature.