Preheat oven to 400°F. Heat the butter, water, milk, sugar and salt in a medium saucepan over medium heat. Bring to a simmer and then reduce heat to low.
Add the flour all at once and stir until it is completely incorporated and clumps into a ball. Continue to mash and stir the dough against the side of the pan for 2 minutes, cooking the flour. Remove from heat and add to the bowl of a stand mixer with a paddle attachment. Let it cool for 2 minutes.
With the mixer running on low speed, slowly add the eggs in 3-4 separate additions, making sure you mix for 30 seconds between each. The mixture will curdle but then come together. In the end, the dough will be sticky and firm enough to hold a stiff peak.
Line two baking sheets with parchment paper and lightly brush with some water, which helps create a humid environment.
Transfer the choux pastry to a piping bag fitted with a ½ inch piping tip. Pipe 28 puffs, each 1 ½” in diameter and ½” tall rounds about 1 inch apart.
Remove the craquelin from the freezer. Cut circles that are the same size as the dough. Place each piece carefully on top of the choux pastry and press lightly to adhere. (If you are baking one sheet at a time, only add the craquelin right before baking)
Bake 1 sheet at a time for 20 minutes, then reduce oven to 350°F and bake for another 10-15 minutes or until golden brown. DO NOT open the oven or they won't puff. Remove and transfer to a cooling rack, letting them cool to room temperature.
Fill the cream puffs by cutting a little hole with the tip of a sharp knife in the bottom. Pipe the chocolate filling into the cream puffs.