|27 tbsp. (13½ oz.)||Plugrà unsalted butter, very cold, divided|
|1¾ cups||whole milk|
|4 tsp.||instant or rapid-rise yeast|
|4¼ cups (21¼ oz)||King Arthur all-purpose flour|
|¼ cup (1¾ oz)||sugar|
|2 tsp.||table salt|
|3½ oz.||bittersweet chocolate, chopped fine|
|1 large||egg, beaten with 1 tbsp water and pinch salt|
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 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 for 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. 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 for at least 45 minutes.
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 your fingertips. Using 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 at least 2 hours or up to 24 hours.
Transfer dough to freezer. After 30 minutes, transfer to lightly floured counter and roll into 16 by 18-inch rectangle with short side of rectangle parallel to counter edge. Using sharp pizza wheel or knife, staring at 1 short side, cut dough into four 4 by 18-inch strips. Then cut dough crosswise into 4 by 4½ inch rectangles (16 rectangles total).
With short side of 1 dough rectangle facing you, spread 1 teaspoon chopped chocolate, end to end, in tidy row, ½ inch from bottom. Stretch and fold bottom third of dough over chocolate to center. Spread 1 more teaspoon chocolate end to end along dough seam. Brush top of folded dough with portion of egg wash. Stretch and fold top third of dough over chocolate until even with bottom edge of dough; gently press seam to seal. Repeat with remaining rectangles.
Place 4 croissants each, seam side down, on 2 parchment-lined rimmed baking sheets at least 3 inches apart. Cover loosely with greased plastic. Let rise at room temperature until nearly double in size, 2½ 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). Place remaining 8 croissants on separate parchment-lined rimmed baking sheet, spaced about 1 inch apart. Wrap tightly with greased plastic and freeze until solid, about 2 hours. Transfer frozen croissants from baking sheet to zipper-lock bag and return to freezer. (Frozen croissants can be stored in freezer for up to 2 months. Arrange on 2 sheets as directed and increase rising time by 1 to 2 hours.)
After 2 hours of rising, adjust oven racks to upper-middle and lower-middle positions and heat oven to 425 degrees. Brush croissants with remaining egg wash. Place croissants in oven and reduce temperature to 400 degrees. Bake for 10 minutes, then switch and rotate baking sheets. Continue to bake until deep golden brown, 8 to 12 minutes. Transfer to wire rack and let cool completely, about 1 hour. Serve.