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

Turn the mixer on low-medium speed to combine ingredients for 1 minute. Slowly pour in the milk while mixing.

Raise the speed to medium and knead the dough for 5 minutes until it feels slightly tacky but soft.

Shape the dough into a ball and place on a floured surface. Cover and refrigerate for 30 minutes.

Roll the dough into a 14×10-inch rectangle on a floured surface. Cover and refrigerate for 4 hours or overnight.

For the butter layer, mix 1 and 1/2 cups softened butter and 2 tablespoons flour until smooth. Shape into a 7×10-inch rectangle and refrigerate for 30 minutes.

Roll the dough out to a 10×20-inch rectangle, place the butter layer in the center, and fold the dough over it. Seal edges.

Roll out again to a 10×20-inch rectangle. Fold lengthwise into thirds for the first turn. If too warm, refrigerate for 30 minutes.

Repeat the rolling and folding process two more times, refrigerating between turns.

After the third turn, cover and refrigerate for 4 hours or overnight.

Roll the dough out to an 8×20-inch rectangle, slice in half vertically, then cut each half into eight 2×5-inch rectangles.

Stretch each rectangle to about 8 inches long and place pieces of chocolate on one end. Roll tightly and place on lined baking sheets.

Cover and rest the croissants at room temperature for 1 hour, then refrigerate for 1 hour or longer if desired.

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

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

Cool on a wire rack for a few minutes before serving. Dust with confectioners' sugar if desired.