Heat the oven to 450 degrees Fahrenheit.
In a medium pan, mix butter, milk, salt, sugar and just under ½ cup of water (100 ml).Set over low heat until the butter has melted and then bring the mixture to a boil.
Remove pan from heat and quick add in flour. Rapidly beat the mixture to make a smooth and glossy dough.
Put the pan back over low heat and beat gently for one 1. The dough should become very thick and will form into a ball.
Put the dough into a stand mixer and let cool until barely warm. Beat in eggs, a little at a time, until a smooth and glossy dough that holds its shape is formed.
Transfer the dough to a large piping backfitted with a 1.5cm plain nozzle. Pipe 20 4cm-diameter round onto a lined baking sheet.
Lightly brush each mound with beaten egg yolk.
Stamp out 20 3-cm discs of the craquelin and place a disc on top of each choux mound
Put baking sheet into over and immediately reduce the oven temperature to 375 degrees Fahrenheit.
Bake for 35 minutes, rotating the sheet after 20 minutes. Choux should be puffed, crisp, and golden brown.
Transfer Buns to a wire rack and make a small hold in the side of each with the tip of a small knife or a metal skewer.