Using a medium saucepan, add water, milk, sugar, salt and butter. Cook over medium heat until the butter has melted, and the mixture starts to boil.
Immediately remove it from the heat. Pour in the flour and mix until a paste form. Place it back on the heat until a film develops on the side of the pan.
Transfer themixture to the bowl of a stand-up mixer. Using a paddle attachment, let it run for a few seconds to release some of the steam and heat.
With themixer running, add 4 eggs, 1 egg at a time. Mix until the dough is smooth and glossy, and the eggs are completely incorporated. Stop mixing from time to time to scrape down the sides of the bowl.
The dough should be thick but should fall slowly and steadily from the beaters when you lift them out of the bowl. If the dough is still clinging to the beaters, add the remaining 1-2 eggs and mix until incorporated. (I like to add beaten egg,half egg at the time, until the mixture falls into ribbons onto itself).
Transfer the dough to a piping bag fitted with a medium plain tip and pipe dollop shapes.
Use a spoon and a little bit of water to smooth out any bumps or points on the dough that remain on the surface.
Bake for 25-30 minutes at 350 degrees or until puffed up and light golden brown. Try not to open the oven door too often during the baking.
Transfer baked puffs to the cooling rock and let them cool completely.
Using a pastry bag fitted with a 230 tip, gently pipe custard into each pastry, using just enough to fill the inside (don't over-stuff them).