In a stand mixer, combine 4 cups of all-purpose flour, ¼ cup granulated sugar, 1 teaspoon of salt, and 2 teaspoons of instant yeast. Gradually mix in 1 and ½ cups of cold whole milk until the dough is smooth and slightly sticky, about 5 minutes. Once combined, wrap the dough in plastic wrap and refrigerate for 30 minutes to allow it to relax.

Prepare the butter layer by beating ⅔ cup of softened unsalted butter with 2 tablespoons of all-purpose flour until creamy and homogenous. Shape this mixture into a rectangle approximately ½ inch thick, wrap it in parchment paper, and refrigerate for another 30 minutes.

On a lightly floured surface, roll the chilled dough into a large rectangle, about ¼ inch thick. Place the prepared butter layer in the center of this dough rectangle, folding the edges over to completely enclose the butter. Seal the edges by pinching them to avoid any leakage, then roll out the dough again into a long rectangle, roughly ¼ inch thick, folding it into thirds like a letter. Chill this folded dough for 30 minutes.

Remove the dough from the refrigerator, roll it out into a rectangle once more, and repeat the folding process into thirds. Chill again for another 30 minutes. Repeat the lamination process two more times.

After completing the final fold, wrap the dough in plastic wrap and refrigerate for at least 4 hours or overnight.

Roll the chilled dough into a large rectangle, about ¼ inch thick, then cut it into rectangular pieces, approximately 4x6 inches each. Place a chocolate baton at one end of each rectangle, roll tightly from that end towards the opposite side, and place the croissant seam-side down on a baking sheet lined with parchment paper.

Arrange the shaped croissants on baking sheets, leaving space for them to expand. Allow them to proof at room temperature for 1 hour. After this, refrigerate them for an additional hour.

Preheat your oven to 400°F (204°C). Brush the tops of the croissants with an egg wash made from 1 beaten egg mixed with 1 tablespoon of milk. Bake for about 20 minutes, rotating trays halfway through.

Once baked, carefully transfer the homemade chocolate croissants to a wire rack and allow them to cool slightly. Dust with confectioners' sugar if desired.