Cut the butter into four 1-Tablespoon slices and place in the bowl of an electric stand mixer with the dough hook attachment. Add flour, sugar, salt, and yeast.

Turn the mixer on low-medium speed for 1 minute to combine. Slowly pour in the cold milk while the mixer is running.

Increase mixer speed to medium and knead the dough for 5 minutes or knead by hand for the same duration.

If the dough is too sticky, add flour 1 teaspoon at a time until the dough is soft and slightly tacky.

Shape the dough into a ball, place on a floured surface, flatten gently, and cover with plastic wrap. Refrigerate for 30 minutes.

Roll the dough into a 14x10-inch rectangle and refrigerate for 4 hours or overnight.

In a separate bowl, beat 1 and 1/2 cups unsalted butter with 2 tablespoons flour until smooth. Shape into a 7x10-inch rectangle on a lined baking sheet and refrigerate for 30 minutes.

Roll out the dough and place the chilled butter layer in the center. Fold the dough over the butter and seal the edges.

Roll the dough into a 10x20-inch rectangle and fold it lengthwise into thirds. Refrigerate for 30 minutes.

Repeat the rolling and folding process for a total of 3 turns, refrigerating between each turn for 30 minutes.

After the last turn, refrigerate the folded dough for 4 hours or overnight.

Roll the dough into an 8x20-inch rectangle. Cut the dough into 16 2x5-inch rectangles.

Stretch each rectangle to 8 inches long and place chocolate pieces at one end. Roll tightly, ensuring the end is on the bottom. Place on lined baking sheets.

Loosely cover and let croissants rest at room temperature for 1 hour, then refrigerate for 1 hour or up to 12 hours.

Preheat oven to 400°F (204°C). Whisk together the egg and milk for the egg wash and brush lightly over the croissants.

Bake for about 20 minutes until golden brown, rotating pans halfway through. If browning too quickly, reduce temperature to 375°F (190°C).

Remove from oven and cool on a wire rack. Dust with confectioners’ sugar if desired.