Preheat the oven to 190°C.
In a saucepan, combine the water, milk, butter, and salt. Bring to boil over medium heat.
Stir in all of the flour with a silicone spatula or wooden spoon. Reduce the heat to low and continue to cook, stirring constantly, until the mixture becomes sticky paste and there’s a visible film of starch on the bottom of the pan, 2 to 3 minutes.
Whisk the eggs. Turn on the mixer, while mixing the flour mixture add the eggs in slow. Continue mixing until totally incorporated, about 4 minutes total. If the dough is too stiff or pulls away too quickly, add another egg to loosen the consistency.
Using the baking dish as a guide, trace two squares with pencil on the parchment, then flip the parchment. Spread the choux evenly inside those two squares, leave out around 1 cm around to border for the pastry to expand. Don't make the squares too thick and place the tray low in the oven.
Bake for 25 to 40 minutes, until puffed and golden brown. (Don’t open the oven during the first 25 minutes, otherwise the choux will depuff.) Transfer the baked choux squares to a wire rack to cool completely.