In the bowl of a stand mixer fitted with the paddle attachment, beat flour, sugar, 1 tablespoon (9 grams) salt, and yeast at low speed just until combined.

In a small bowl, combine warm milk and ⅓ cup (80 grams) warm water. With mixer on low speed, add milk mixture to flour mixture in a slow, steady stream; beat for about 1 minute. Add butter, 2 tablespoons (28 grams) at a time, beating until combined after each addition; beat for 4 minutes. Increase mixer speed to medium-low, and beat until dough passes the windowpane test, about 3 minutes. (See Note.) (If your mixer seems to be having a hard time toward the end, finish kneading by hand.)

Divide dough in half; shape each half into a ball. Place each dough ball on a parchment paper-lined baking sheet, and wrap in plastic wrap. Refrigerate overnight, or let stand at room temperature for 30 minutes and then refrigerate for 3½ hours.

Punch down each dough ball. Cover 1 dough ball. Roll uncovered dough ball into a 12x8-inch rectangle; repeat with remaining dough. Quickly place Butter Block directly on top of 1 dough rectangle. Cover with remaining dough rectangle, creating a “butter sandwich.” Pinch together edges of top and bottom doughs to seal. Working quickly and carefully, roll into a 24x9-inch rectangle. (Be careful not to tear dough or allow any butter to leak out.) Trim about ½ inch off short sides so butter layer is exposed. Fold dough in half so short sides meet. Pat fold to mark center. Unfold dough, and pull one short side to center. Pull opposite side to center so short ends are touching. Pinch seams together; fold along center seam, like closing a book. (At this point, you should have 4 layers of dough.) Return to pan, and wrap in plastic wrap. Freeze for 15 minutes; refrigerate for 20 minutes.

Roll dough into a 30x9-inch rectangle. Trim about ¼ to ½ inch off short sides so butter layers are exposed. Fold dough in half so short sides meet. Pat fold to mark center. Unfold dough, and pull one short side to center. Pull opposite side to center so short ends are touching. Pinch seams together; fold along center seam, like closing a book. (Your dough will be thinner this time.) Return to pan, and wrap in plastic wrap. Freeze for 15 minutes; refrigerate for 20 minutes.

Line 2 baking sheets with parchment paper.

Bring a small ovenproof saucepan of water to a boil over medium-high heat. Place pan in back right corner of cold oven. Keep oven door closed.

Roll dough into an 18x12-inch rectangle. Trim short sides so butter layers are exposed. Trim ¼ inch off long sides. Cover dough, and freeze for 8 to 10 minutes.

Working quickly, cut dough into 4 (12x4-inch) rectangles. Cut each rectangle diagonally to create 8 triangles. Make a 1-inch cut from center of 4-inch base of each triangle. Very gently stretch triangle to relax dough. Fold center corners of base toward sharpest point of triangle, and roll dough, jelly roll style, toward sharpest point. Place, point side down, about 3½ inches apart on prepared pans. Let rise in prepared oven for 2½ hours. (Try not to place pans directly over steamy water. Do not open oven door while proofing. Croissants will double in size, have a little separation in their layers, and be light as a feather.)

Remove croissants and saucepan of water from oven. Loosely cover croissants with greased plastic wrap. (Do not secure plastic wrap. You don’t want to put any weight or pressure on proofed dough.) Refrigerate until oven is preheated.

Position oven racks in center and top of oven. Preheat oven to 425°F (220°C).

In a small bowl, whisk together egg and remaining ¼ teaspoon salt. Brush croissants with egg wash.

Bake on center rack for 5 minutes. Place a sheet of foil on top rack, and reduce oven temperature to 375°F (190°C). Bake for 5 minutes more. Rotate pans, and bake for 8 to 10 minutes more.