Preheat the oven to 425F (218C) convection (400F/204C conventional) and line 2-3 baking sheets with parchment paper.
In a medium sauce pan, heat the water and butter over medium heat until the butter is fully melted.
1 cup (240mL) water, 1/2 cup (115g) unsalted butter
Remove it from the heat, add in the flour and salt, and mix it together until it is fully combined. Place it back on the heat and continue mixing it for 1-3 minutes until the dough turns into one lumpy mixture (sticks to itself in a ball).
1 cup (125g) all-purpose flour, 1/2 tsp salt
Remove it from the heat and place it in the bowl of a stand mixer (you can do it by hand too). Use a paddle attachment turn it on low speed to help it cool a little before adding the eggs. Once it is warm to the touch and not hot enough to scramble the eggs, add one egg to the mixture and turn the mixer on medium speed. It will look like it is splitting- just let it fully combine, then add in another egg.
4-6 large eggs
Make sure to add one egg at a time and check the consistency of the dough before each one. It should be pipe-able but not at all runny. When the mixer stops and the paddle is lifted, the dough should form a 'v' shape hanging off of the paddle attachment.
Place the dough into a piping bag and pipe out a 1-2-inch circles. Remember they double-triple in size. Beat the egg for the egg wash and lightly brush each choux.
1 large egg
Remove the craquelin from the fridge and cut it into circles, the same size as the choux pastry. Place one circle on top of each choux, pressing only lightly so it sticks.
Place them in the oven and immediately turn the oven down to 375F (191C) and bake them for 25 minutes. Lower the oven to 325F (163C) and bake for another 15-20 minutes (depending on size) until golden brown. I like them crispy so I leave them in as long as possible to dry them out without burning.
Remove them from the oven and allow them to cool.