Add the milk, water, salt, sugar, and butter to a medium saucepan, then 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 add the sifted flour all at once.
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 to medium-high heat and continue to stir and keep it moving to dry out the dough.
Once you see a browning layer on the bottom of the pan, the dough is dry enough and you can remove it from the heat.
Put the dough immediately into the bowl of your stand mixer with the paddle attachment and begin to mix to cool it down, this should take about 30 seconds.
Add the eggs one at a time, letting each egg incorporate fully before the next. Mix on low until the dough is 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 pastry bag with a large round tip, Christophe is using an Ateco 809 tip. Fill the piping bag with the dough.
On to a prepared baking tray, pipe the pâte à choux rounds slightly smaller than your desired final size, as they will “puff” and expand in the oven. Chef Christophe is using a "silpain" baking mat, but parchment paper will work as well.
Gently smooth the top of the cream puff with a wet finger.
Bake at 350°F (176°C) for 25-30 minutes until golden brown. Let them cool on a wire rack to room temperature for about 30 minutes.