Preheat oven to 200°C (400°F). Line a baking sheet with parchment paper.
Combine the butter, water, milk, salt, and granulated sugar together in a medium saucepan over medium heat. Stir until the butter has melted. Bring mixture to a simmer. Once simmering, reduce heat to low and add the flour all at once. Stir until the flour is completely incorporated and a thick dough clumps into a ball.
Mash the dough ball against the bottom and sides of the pan for 1 minute to gently cook the flour.
Remove from heat and transfer to the bowl of a stand mixer fitted with a paddle attachment or, if using a handheld mixer, a large mixing bowl. Allow to cool down for a few minutes before adding the eggs.
Beat the eggs separatly and gradually add it to the dough, in 3-4 separate additions mixing for 30 seconds between each. Stop adding when the choux pastry has reached the desired texture: shiny, thick, and smooth with a pipeable consistency.
Transfer your dough into a piping bag (I use a 1A nozzle). Pipe small balls onto your baking sheet with a little space between each ball. I make about 12 with this amount
If you don't want to use it immedietly. Cover and refrigerate for up to 3 days.
Once they've all been piped, wet your finger and dab the top of each ball to help with the shape. Gently flatten any pointy bits.
Gently brush with egg wash. (optional, if you don't have any of the whisked eggs left you can omit this step as it's not necessary.)
Bake in the prehetead oven 200°C (400°F) for 20min. Then 10 more minutes on 175°C (350°F). Do not open the oven in this 30min as the pastries cook, as cool air will prevent them from properly puffing up. Once this 30min have passed open the oven, prick each shell with a skewer or toothpick and let them dry out in the oven for 5 more minutes.
Remove from the oven and transfer to a cooling rack. Allow to cool completely before filling.