Combine softened butter, sugar and flour together in a mixing bowl with a wooden spoon or stiff spatula until a soft dough is formed.
Roll out the dough with a rolling pin between two sheets of silpat or parchment paper until the dough is very thin, about 2-mm thick. (Note that you may need to divide the dough into 2 portions if your silpat is half sheet pan size 46x33-cm).
Gently peel off the top layer of the silpat / parchment paper and use a 3.5-cm round cutter to cut as many rounds as possible. Freeze the croustillant in the freezer until needed. (See Note 1.)
Salted Egg Yolk Paste
Place milk and butter in a small pot. Cook over low heat, stirring frequently with a spatula, until the butter has melted. Turn up the heat and bring the mixture to a boil.
Remove from heat and use a wire whisk to whisk in the salted egg yolk powder until a thick paste is formed. Set aside.
Salted Egg Yolk Pastry Cream
Whisk egg yolks and sugar with a wire whisk in a small bowl. Whisk in the flour. Set aside.
Heat milk and salt in a pot over high heat until it starts to simmer. Pour the hot milk in a thin stream into the egg-yolk mixture while whisking continuously until all the hot milk is added, making sure to scrape the bottom of the bowl to mix everything thoroughly.
Pour the milk and egg yolk mixture back into the pot and cook over medium heat, whisking constantly. The mixture will be thin and frothy with lots of bubbles on the surface but it will soon thicken. When the mixture starts to thicken, remove the pot from heat and whisk rigorously until the mixture is homogenous. Return the pot back to the heat and whisk constantly until the pastry cream boils - big bubbles will start popping out of the pastry cream.
Remove the pot from heat and whisk in the Salted Egg Yolk Paste. Scrape the mixture into a clean, dry bowl (the larger the better, so it cools down faster.) If there are lumps, strain the pastry cream through a sieve. Press a piece of parchment paper or plastic wrap onto the surface of the pastry cream and chill completely in the refrigerator, about 1 hour.
Preheat oven to 180C / 355F. Line baking sheets with parchment paper or silpat. Dip the 3.5-cm round cutter (the one used to cut the croustillant) into flour and make markings on the baking sheet. Have all the ingredients measured and ready.
Combine milk, water, butter, sugar and salt in a pot. Heat the mixture over low heat until all the butter has melted. Turn up the heat to high and bring the mixture to a rolling boil, stirring occasionally with a wooden spoon. Immediately remove the pot from heat, add in the flour in one go and stir quickly and vigorously with a wooden spoon to mix in all the flour to form a dough.
Once all the flour has mixed in, return to medium-low heat and continue cooking the choux, mixing with the wooden spoon to dry out the dough a little bit. The dough is ready when a thin film of dough forms at the bottom of the pot. Do not scrape the film of dough formed at the bottom of the pot up. Remove the pot from heat and transfer the dough into a clean mixing bowl and let cool for 5 minutes, stirring occasionally.
Using an electric mixer on low speed or with the wooden spoon, add in the beaten eggs in 4 parts, mixing well after adding each part of egg. The choux batter is ready when it will drop down from the beaters or the wooden spoon and form a "V" shape. If the choux batter is still too stiff, beat in milk, tablespoon by tablespoon until the right consistency is obtained.
Transfer the choux into a piping bag fitted with a round tip (maximum 1.5-cm big). Pipe 3.5-cm mounds of choux onto the baking sheet.
Remove the frozen croustillant from the freezer and working quickly, use an offset spatula to loosen the croustillant rounds and place one on each piped choux, pressing it down slightly.
Bake at 180C / 355F for 20 minutes, until the dough is well-risen - do not open the oven door at all during this period to check on the choux. Reduce the heat down to 150C / 300F, open the oven door to release the steam and close the oven door quickly. (Do this quickly so that the oven temperature don't drop too much.) Bake for another 20 minutes until the choux is dry, crisp and golden brown.
Remove the choux from the oven. Turn the choux over and use a piping tip to poke holes to release excess steam. Transfer to a wire rack and let cool completely before filling with pastry cream.
Whip the cream (either with a hand mixer or manually with a wire whisk) until semi-stiff peaks are formed.
Remove the pastry cream from the refrigerator - it will be quite firm at this point so use a wire whisk and whisk gently to make it smooth. Don't whisk too much as the pastry cream will become watery. Gently fold in the whipped cream.
Transfer the pastry cream into a piping bag and fill the choux with pastry cream.
If you can't peel off the silpat / parchment paper without the croustillant sticking to the top side, freeze the croustillant briefly for 15 minutes and try again.
To ensure the croustillant stays crunchy, the cream puffs are best eaten as soon as possible after they are filled, so try to fill them only at the last minute. Both the choux and salted egg yolk pastry cream (without the whipped cream) can be made a day ahead. Store the choux in an airtight container at room temperature and the pastry cream in the refrigerator. If the choux is soft the next day, they can be re-crisped in a 175C / 350F oven for 5 to 10 minutes. Fold in the whipped cream into the pastry cream just before filling.