Preheat oven to 400°F/204°C. Line two baking sheets with parchment paper. Set aside.
Prepare the choux pastry dough. In a medium saucepan, combine the butter, water, milk, salt, and granulated sugar together in a medium saucepan over medium heat. Stirring occasionally. Bring mixture just to a boil then reduce to low heat and add in flour all at once. Continue stirring until the flour is incorporated and the mixture pulls away from the sides of the pan, forming a smooth dough ball. Remove from heat and transfer to the bowl of a large mixing bowl. Allow to cool down for a few minutes before adding the eggs.
Use a stand mixer (or a handheld mixer) fitted with a paddle attachment to beat 4 eggs and vanilla extract on a low speed, one at a time, beat well after each addition until fully incorporated. Once all of the eggs have been added, scrape down the bottom and sides of the bowl and mix again to ensure everything is fully incorporated, an additional 5 to 10 seconds. The dough will become smooth and shiny and can form a thick ribbon when pulled up.
Transfer the dough to a piping bag fitted with a 1/2-inch round tip (I use Wilton 1A tip). Use immediately or refrigerate for up to 2 days in a piping bag or with a piece of plastic wrap pressed onto the surface.
Bake cream puffs. Pipe the choux pastry dough onto prepared baking sheets into 18 small mounds (or 14 medium mounds). Keep 2 inch apart between the puffs. Smooth the peaks out with your finger lightly dipped with water. Bake at 400˚F for 15 minutes then reduce temperature to 350˚F and bake 20-22 min longer or until golden brown on top. Make sure not to open the oven while they are baking or cream puffs will deflate. Remove from oven and transfer to a wire rack to cool completely before adding filling.
Fill cream puffs. Slice the puffs in half and pipe pastry cream onto the bottom of the puff using a pastry bag fitted with a piping tip (I use Wilton 1M), then cover with the top half. You can also poke a hold on the bottom of the puff and gently pipe until filled. Dust with powdered sugar and serve. Cream puffs are best served cold.