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.