Preheat oven to 375°F. Line a baking sheet with parchment paper.
Place water, milk, salt, sugar, and butter in a saucepan and heat over medium heat. Cook, stirring, for 3-4 minutes or until butter melts and mixture just comes to the boil.
Add all the flour to the butter mixture at once, beat until well combined.
Place over low heat and cook, stirring, for 2-4 minutes or until the mixture forms a ball and begins to come away from the side of the saucepan.
Remove from heat and transfer to the bowl of a stand mixer fitted with a paddle attachment or a large mixing bowl if using a handheld mixer. Set aside for 5 minutes to cool slightly.
Mix the dough while adding the eggs in 3-4 separate additions, mixing well after each addition. Beat until the mixture just falls from the spatula but still holds its shape.
Pipe the cream puffs onto the baking tray or spoon dollops, if using a teaspoon.
Brush the tops with egg wash. Bake in a preheated oven for 25-35 minutes or until the cream puffs are puffed and golden. Remove the cream puffs from the oven and transfer to a wire rack to cool.