Pain au Chocolat (Chocolate Croissant)

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Ingredients :
1 tbsp. yeast
1/2 cup milk
1/3 cup sugar
1 1/2 cups plus 2 tbsp unsalted butter, barely softened; plus 3 tbsp melted and cooled
1 tbsp powdered milk
1 tbsp kosher salt
4 cups flour
18 1/2″-wide x 3″-long bittersweet chocolate bars
1 egg, mixed with 1 tbsp water, for egg wash
Instructions
Stir together yeast and ½ cup water heated to 115° together in the bowl of a stand mixer fitted with a dough hook; let sit until foamy, about 10 minutes. Stir in milk, sugar, 3 tbsp. melted butter, powdered milk, and salt; add flour. Mix on medium speed until a stiff dough forms, about 5 minutes. Transfer to a work surface and form into a thick square; wrap in plastic wrap and refrigerate for 2 hours.
Place remaining butter on a sheet of plastic wrap; cover with another sheet. Using a rolling pin, pound and shape butter into a 6, ½”-thick square; set aside. Using rolling pin, roll dough into an 16” square on a lightly floured work surface. Unwrap butter square, and place on dough so its corners line up with the middle point of each side of dough square; fold dough corners over butter so they meet in the center. Roll dough into a 12” x 9” rectangle, and then tri-fold dough like a letter. Roll the dough into a 12” x 9” rectangle and repeat folding. Wrap in plastic wrap; chill for 30 minutes. Repeat rolling and folding dough twice more; chill for 1 hour.
Roll dough into a 20” x 14” x ¼” thick sheet; halve lengthwise. Cut each half into about 9 triangles, about 3 ½”-wide at their base; cut a ½”-deep slit in the middle of each wide base. Place 1 chocolate bar parallel to the base near the cut; holding the tip of the opposite corner down, roll the base over the chocolate toward the tip until it forms a tight roll. Place croissant on a parchment paper-lined baking sheet, tip side down, and brush with egg wash. Repeat with remaining triangles, bars, and egg wash. Let croissants sit until doubled in size, about 2 ½-3 hours.
Heat oven to 375°. Working with one baking sheet at a time, brush croissants with more egg wash; bake until deep golden brown, about 20 minutes.
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