In a Medium saucepan, combine water, milk, butter, sugar and salt. Bring to a boil over medium heat then remove from heat and stir in 1 cup flour all at once with a wooden spoon.
Once the flour is incorporated, place back over medium heat about 1 1/2 to 2 minutes stirring constantly (to release extra moisture and partially cook flour), or until dough comes together into a smooth ball and a thin film forms on bottom of pan.
Transfer to a large mixing bowl and beat using an electric hand mixer on medium speed for 1 minute to cool the mixture slightly. Add eggs, 1 at a time, allowing each egg to fully incorporate between additions. Beat until dough is smooth and forms a thick ribbon when pulled up.
Put the pastry dough into a tipless piping bag (or zip-top bag) and pipe eclairs onto a parchment-lined baking sheet. Pipe 18-20 (4” long and 3/4” wide) strips, keeping them about 2" apart. Bake at 425˚F for 10 minutes. Without opening oven, reduce temp to 325˚F and, bake 30 minutes longer or until golden brown. Transfer to wire rack to cool while making pastry cream.