In a pot over medium-high heat, add the water, milk, butter, salt and sugar.
As soon as you see it it's on a rolling boil add the flour and quickly mix with a spatula. When the dough comes together as one large ball transfer it to a bowl where you're later going to mix using an electric mixer or a spatula. It's important to remove the dough from the pot at this point to prevent it from cooking further.
The dough needs to cool down completely before adding the eggs or the choux pastry won't expand in the oven. You can let it cool on its own or use an electric mixer with the paddle attachment to help it cool down faster.
Once the dough is cool add an egg and mix. If you're doing this by hand, use a spatula. First, it will look like it's not coming together but it will after a while. If you're using an electric mixer make sure to scrape down the sides every once in a while to help the dough incorporate.
At this point, make the test we talked about in the post: lift a bit of dough with a spatula and let it drop on its own. If it falls on its own and leaves behind a "V" of dough hanging from the spatula then you don't need to add more egg.
If you're still not there then whisk an egg in a small bowl. Add about ¼ of it and make the "V" test again. Repeat until the test comes out correctly and remember to always scrape down the sides of the bowl.
Fill a piping bag with the dough and make small 3cm diameter buns on a baking tray lined with baking paper or a silicone mat. Leave about 4-5cm between each; they expand a lot in the oven!
Take the choux buns to a preheated oven at 180°C/350°F for 15-20min, they should be slightly golden.
Lower the temperature to 150°C/300°F and leave them in there for 10 more minutes. In all of this time don't open the oven because they're really sensitiv and will deflate.
Remove them from the oven and place them on a cooling rack. Don't fill them while still warm because the filling will melt and the choux buns will become soggy.
Now that they're cooked you can freeze them for up to 3 months and then before serving, you place them in the oven at 140C until they're back to normal.