Pâte à choux is yet another one of those recipes in my repertoire that I regard as "essential". If you want to be a well-rounded baker, this is something you must learn. Its the definition of simple and even more so, one of versatility. Make eclairs, profiteroles, gougères, St. Honore, doughnuts, cream puffs, churros, etc, all by altering this recipe or adapting it with others. The very essence of successful baking, after all, is combining ordinary recipes for a extraordinary result. —PieceOfLayerCake
Combine the milk, water, butter, salt and sugar in a medium-sized sheet pan and set over medium heat. Allow the ingredients to melt together, stirring occasionally. Bring the mixture to a boil.
Remove the pan from the heat and dump in the flour, all at once. Vigorously stir the ingredients together and allow the flour to absorb the liquid. Place the pan back on medium heat and continue to stir the mixture, allowing the heat to cook it until it comes together in a smooth, slightly glossy mass.
Remove the mixture from the heat and continue to stir it, releasing the intense heat. When the mixture has cooled slightly, but the steaming has subsided, begin to add the egg gradually, stirring completely after each addition. Continue adding egg, stopping after you're left with a very smooth, creamy batter (think of very creamy mashed potatoes).
Your pâte à choux is now ready for whatever application you intend it to fulfill. For example, if making simple cream puffs, preheat the oven to 375℉. Pipe the pâte à choux into even, 2" rounds. Use a moistened finger to rub away any peaks. Bake the rounds for 25 - 30 minutes, or until crisp and richly golden-brown. Transfer immediately to a wire rack to cool.