1. Put the cold butter in the bowl of a stand mixer with the paddle attachment and beat on medium-high speed until there are no lumps in the butter, about 15 seconds. (If you don’t have a stand mixer, whack the butter with a rolling pin, turning it a few times, until it’s a cold paste.) Lay a piece of plastic wrap on the counter and place the butter in the middle. Enclose the butter and shape it into a 4- by 3-inch (10 by 8cm) rectangle. Chill the butter for 20 to 30 minutes.
2. Remove the dough from the refrigerator. Roll the dough on a lightly floured countertop, so it forms a diamond shape with four flaps – two on top, two on the bottom, leaving the dough raised a bit in the center. (See the photo in the post.)
3. Unwrap the chilled rectangle of butter and place it in the center. Fold the flaps over the butter, sealing the butter completely, and whack the dough with a rolling pin to flatten it out. Roll the dough into a 12- by 9-inch (30 by 22cm) rectangle.
4. Lift up one-third of the left side of the dough and fold it over the center. Then lift the right side of the dough over the center, to create a rectangle. Take the rolling pin and press down on the dough two times, making an X across it. Mark the dough with one dimple with your finger to remind you that you’ve made one “turn”, wrap it in plastic wrap, and chill the dough for 45 to 60 minutes.
5. Do the next turn of the dough the same way, rolling and folding the dough again, making 2 dimples with your finger in the dough, then chill it for another 45 to 60 minutes.
(The resting period between steps #4 and #5 can be longer in case you have other things to do. Feel free to let it rest a couple of hours between each turn. It’ll be fine.)
6. Do the last turn and folding of the dough and let it chill for an hour. (The dough can be chilled overnight at this point, or frozen.)
7. To shape the croissants, line a baking sheet with parchment paper or a silicone baking mat. Unwrap the dough and roll it out on a lightly floured countertop until it’s a 12- by 9-inch (30 by 22cm) rectangle. Trim the edges off with a sharp chef’s knife and cut the dough into 3 rectangles, then cut each rectangle diagonally, making 6 triangles (as shown in the post.)
Take one triangle and roll to lengthen it to 11 inches (28cm) long. Starting at the wide end, roll the croissant up toward the point, not too-tightly. Set it point-side-up on the baking sheet and roll the rest of the croissants the same way.
8. Cover the baking sheet with a large plastic bag (such as a clean trash bag), close it, and let the croissants proof in a warm place until the croissants are nearly doubled and puffed up, which will take 1 1/2 to 2 hours. (If you wish, you can chill the rolled croissants overnight. Take them out of the refrigerator and let them proof in a warm place, as indicated.)
9. Preheat the oven to 400ºF (200ºC.) Mix the egg with a pinch of salt and brush each croissant with the glaze. Bake the croissants for 5 minutes, then reduce the heat of the oven to 350ºF, and bake for 20 to 25 minutes, until browned. Some butter may seep out during baking, which is normal.