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1
Make the croissant dough. Dissolve the yeast in the warm milk. Sift the all-purpose flour, then incorporate the sea salt, sugar, very soft butter, dry milk powder, two-thirds of the warm still mineral water, and the yeast dissolved in milk. Knead the mixture briefly. Add more of the remaining water if the dough appears to be too firm. Transfer the dough to a bowl, cover it with plastic wrap, and keep it at room temperature (ideally at 72u00b0F [20u00b0C]) for between 1 and 1 1/2 hours, or until the dough has doubled in volume.
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2
Punch down the dough to its initial volume and cover it with plastic wrap again. Refrigerate for 1 hour. Punch it down again and place in the freezer for 30 minutes.
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3
Remove the dough from the freezer. Pound half the cold butter with a rolling pin to soften, then knead butter with hands forming a rectangle. Sprinkle a work surface with flour and roll out a long rectangle of dough; it should be three times longer than it is wide. Place the butter against the lower edge of the dough. With the palm of your hand, pull the butter over the dough until it covers two-thirds of it. Wrap the remaining third of the dough over the two-thirds covered in butter. Fold the lowest third of the buttered part over the other third, then wrap the upper third over it. Place the dough in the freezer for 30 minutes, then refrigerate for 1 hour.
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4
Roll out the dough as before and proceed in the same way with the other half of the butter. Place the dough in the freezer again for 30 minutes, then refrigerate for 1 hour.
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5
Sprinkle a work surface with all-purpose flour, and roll out the dough to a thickness of 1/8 inch (3 mm). Using a sharp knife, cut out isosceles triangles 8 inches (20 cm) high and with a base measuring 4 1/2 (12 cm) inches. Place each triangle on the work surface with the base pointing toward you. Roll it up on itself, then curve the ends into a crescent, or croissant, shape. Line two baking sheets with parchment paper. As you finish shaping the croissants, place them on the lined sheets, spacing them 2 inches (5 cm) apart. Set them aside at room temperature to proof for about 1 1/2 hours.
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6
Preheat a convection oven to 410u00b0F (210u00b0C).
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7
Make the glaze. Whisk together the egg yolk, eggs, and salt in a bowl. Using a pastry brush, coat the croissants with the glaze. Place the croissants in the oven and immediately reduce the oven temperature to 350u00b0F (180u00b0C). Bake for 20 minutes. Remove from the oven and transfer the croissants to wire racks to cool.
