Stir together yeast and ½ cup water heated to 115°F 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.

On a lightly floured work surface, roll dough into a 16″ square.

Unwrap butter square and place on dough so its corners line up with the middle point of each side of the dough square; fold dough corners over butter so they meet in the center.

Roll dough into a 12″ x 9″ rectangle, then tri-fold the dough like a letter.

Wrap in plastic wrap and 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 3½″-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 2½–3 hours.

Preheat oven to 375°F.

Working with one baking sheet at a time, brush croissants with more egg wash; bake until deep golden brown, about 20 minutes.