In a stand mixer bowl, combine the flour, sugar, salt, and instant yeast. With the dough hook running, slowly pour in the cold milk and knead for about 5 minutes until soft and smooth but slightly tacky.

Cover the bowl and refrigerate for 30 minutes.

Beat the softened butter and 2 tablespoons of flour until smooth. Shape into a 7x10-inch rectangle and refrigerate for 30 minutes.

Roll the chilled dough into a 14x10-inch rectangle and place the butter in the center. Fold the dough over to encase the butter and seal the edges.

Roll the dough into a 10x20-inch rectangle, fold into thirds and chill for 30 minutes. Repeat two more times. After the last fold, refrigerate for 4 hours or overnight.

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

Gently stretch each rectangle to about 8 inches long, place a chocolate baton at one end, and roll up tightly. Arrange on lined baking sheets seam-side down.

Let croissants proof at room temperature for 1 hour, then refrigerate for 1 hour.

Preheat the oven to 400°F (204°C). Whisk together the egg and milk, brush over each croissant, and bake for 20 minutes until golden.

Let cool slightly on a wire rack and dust with confectioners’ sugar if desired.