Melt 3 tablespoons butter in medium saucepan over low heat. Remove from heat and immediately stir in milk (temperature should be lower than 90 degrees). Whisk in yeast; transfer milk mixture to bowl of stand mixer. Add flour, sugar, and 2 teaspoons salt. Using dough hook, knead on low speed until cohesive dough forms, 2 to 3 minutes. Increase speed to medium-low and knead for 1 minute. Remove bowl from mixer and cover with plastic wrap. Let dough rest at room temperature 30 minutes.
Transfer dough to parchment paper–lined rimmed baking sheet and shape into 10 by 7-inch rectangle about 1 inch thick. Wrap tightly with plastic and refrigerate for 2 hours.
BUTTER BLOCK: While dough chills, fold 24-inch length of parchment in half to create 12-inch rectangle. Fold over 3 open sides of rectangle to form 8-inch square with enclosed sides. Crease folds firmly. Place 24 tablespoons cold butter directly on counter and beat with rolling pin for about 60 seconds until butter is just pliable but not warm, then fold butter in on itself using bench scraper. Beat into rough 6-inch square. Unfold parchment envelope. Using bench scraper, transfer butter to center of parchment, refolding at creases to enclose. Turn packet over so that flaps are underneath and gently roll until butter fills parchment square, taking care to achieve even thickness. Refrigerate at least 45 minutes.
LAMINATE: Transfer dough to freezer. After 30 minutes, transfer to lightly floured counter and roll into 17 by 8-inch rectangle with long side parallel to edge of counter. Unwrap butter and place in center of dough. Fold sides of dough over butter so they meet in center. Press seam together with fingertips. With rolling pin, press firmly on each open end of packet. Roll out lengthwise into 24 by 8-inch rectangle. Starting at bottom of dough, fold into thirds like business letter into 8-inch square. Turn dough 90 degrees counterclockwise. Roll out lengthwise again into 24 by 8-inch rectangle and fold into thirds. Place dough on sheet, wrap tightly with plastic, and return to freezer for 30 minutes.
Transfer dough to lightly floured counter so that top flap opens on right. Roll out dough lengthwise into 24 by 8-inch rectangle and fold into thirds. Place dough on sheet, wrap tightly with plastic, and refrigerate for 2 hours or up to 24 hours.
SHAPE: Transfer dough to freezer. After 30 minutes, transfer to lightly floured counter and roll into 18 by 16-inch rectangle with long side of rectangle parallel to edge of counter. Fold upper half of dough over lower half. Using ruler, mark dough at 3-inch intervals along bottom edge with bench scraper (you should have 5 marks). Move ruler to top edge of dough, measure in 11/2 inches from left, then use this mark to measure out 3-inch intervals (you should have 6 marks). Starting at lower left corner, use sharp pizza wheel or knife to cut dough from mark to mark. You will have 12 triangles and 5 diamonds; discard scraps. Unfold diamonds and cut into 10 triangles (making 22 equal-size triangles in total).
Position 1 triangle on counter. (Keep remaining triangles covered with plastic.) Cut 1/2-inch slit in center of short side of triangle. Grasp triangle by 2 corners on either side of slit and stretch gently, then stretch bottom point. Place triangle on counter so point is facing you. Fold down both sides of slit. Roll top of triangle partway toward point. Gently grasp point with 1 hand and stretch again. Resume rolling, tucking point underneath. Curve ends gently toward each other to create crescent. Repeat with remaining triangles.
Place 12 croissants on 2 parchment-lined rimmed baking sheets at least 21/2 inches apart. Lightly wrap with plastic. Let stand at room temperature until nearly doubled in size, 21/2 to 3 hours. (Shaped croissants can be refrigerated for up to 18 hours. Remove from refrigerator to rise and add at least 30 minutes to rising time.).
After croissants have been rising for 2 hours, adjust oven racks to upper-middle and lower-middle positions and heat oven to 425 degrees. In small bowl, whisk together egg, water, and pinch salt. Brush croissants with egg wash. Place croissants in oven and reduce temperature to 400 degrees. Bake for 12 minutes, then switch and rotate baking sheets. Continue to bake until deep golden brown, 8 to 12 minutes longer. Transfer to wire rack and cool about 15 minutes. Serve warm or at room temperature.
TO MAKE AHEAD: After shaping, place 10 croissants 1 inch apart on parchment-lined sheet. Wrap with plastic and freeze until solid, about 2 hours. Transfer to zipper-lock bag and freeze for up to 2 months. Bake frozen croissants as directed from step 8, increasing rising time by to 2 hours.