Preheat the oven to 360°F and prepare two sheet pan with parchment paper or a "silpain" baking mat.
Add the milk, water, salt, sugar, and butter to a medium saucepan and bring it to a boil. It is important to make sure that the butter is melting at the same rate the milk is heating so that the milk doesn't boil before the butter is melted. Slowly moving it around with a wooden spoon helps distribute the heat evenly.
Once the mixture has come to a boil, turn off the heat and sift in the flour.
Begin stirring immediately with a wooden spoon, and continue to stir for 1 minute until the flour is fully incorporated, it will look a lot like mashed potatoes. Turn the heat back on medium-high and continue to stir and keep it moving to dry out the dough.
Once you see a browning layer on the bottom of your saucepan, the dough is dry enough and you can remove it from the heat. This is important, as it will help your eclairs “puff” and ensure that they have a hollow center for the cream filling.
Put the dough immediately into the bowl of a stand mixer with the paddle attachment and begin to mix to cool it down, this should take about 30 seconds. When you can place your hand on the side of the bowl without it feeling too hot, it’s done.
Add the eggs one at a time, letting each egg incorporate fully before the next. The dough should be fully combined with no spots of dry flour, and stiff enough to hold its shape but pull to a soft peak when picking up the spoon.
Prepare a piping bag with a large star or large round tip. Fill the piping bag with the dough. I prefer a star tip for pretty lines on top of my eclairs.
Pipe the pâte à choux in equal-length tubes, onto a lined baking sheet, about 3-4” long, and 2” apart. Pro-Tip: Use a cake smoother or a ruler to pipe your eclairs uniformly.
Gently smooth the top of the eclair with a damp fingertip.
Bake the eclair shells at 360°F for 25-30 minutes until golden brown.
Cool the eclair shells completely on a wire rack for about 30 minutes.