Continue with the choux pastry by sifting flour then measure and set aside
Place butter, water, milk, salt, and sugar into a saucepan over medium heat
As soon as the mixture starts to boil remove from heat, add the flour in one go, and whisk vigorously with a Rubber spatula until it forms one smooth ball and flour bits are not visible Place the saucepan back on the stove and cook the dough for about 3 min over medium heat mixing constantly. It is ready when a sort of skin forms on the bottom of the saucepan
Transfer the dough into a bowl and let it cool slightly before mixing the egg in. Only add a small amount of egg at a time, consistently whisking after each addition. You should reach a consistency which is not too runny, not too firm but just fine to pipe
Pipe 20 equal-sized buns on the baking mat leaving enough space in between them, as they will puff, then place the tray into the freezer for 10 min at least
Pre-heat oven to 200 C / 392 F
Take the craquelin sheet out from the freezer and with the help of a cookie cutter cut out 20 circles, slightly larger in diameter than the choux buns you just piped
Take the choux buns out of the freezer and place one craquelin disk on each of them. The craquelin disk should be frozen and very firm at this point, which makes the process of cutting as well as placing them on top of the choux bun easy
Pop the tray with the choux buns back to the freezer for 15 min
Lower oven temperature to 170 C / 338 F and bake the choux for 40 min. Do not open the oven door in the first 30 min. If you are baking it the first time, it is a good idea to take one choux bun out of the oven at 40 min and check its consistency. If it is golden in colour, nicely puffed up, does not deflate at room temperature and the middle of the ball is not raw (it is ok if it's a bit wet, it will dry while cooling down) then it is considered to be done
Cool either on the perforated air mat or if baked on parchment paper, then cool them on cooling rack before filling them