In a large mixing bowl, combine all-purpose flour, granulated sugar, salt, and instant yeast. Slowly add cold whole milk while mixing until a soft dough forms. Knead on a floured surface for about 5 minutes until smooth. Lightly shape into a ball, place in a greased bowl, cover with a damp cloth, and refrigerate for 30 minutes.

While the dough chills, create the butter layer by beating softened unsalted butter with all-purpose flour until smooth. Shape this mixture into a rectangle about 6x8 inches. Wrap it in plastic wrap and chill it in the fridge for 30 minutes.

After chilling, remove the dough from the refrigerator and roll it out on a floured surface into a large rectangle, roughly 12x18 inches. Place the chilled butter layer in the center and fold the dough over to encase it completely.

Roll the encased dough into a rectangle around 12x24 inches. Fold the dough into thirds like a letter, sealing the edges tightly again. Chill in the refrigerator for 30 minutes to firm it up. Repeat this rolling and folding process two more times, chilling for 30 minutes between each turn.

Once the dough is well-chilled, roll it out to a large rectangle about 12x18 inches. Cut the dough into long triangles with a base of about 4 inches. Place a piece of chocolate baton towards the wide end of each triangle, then roll them tightly.

Arrange the shaped chocolate croissants on lined baking sheets, cover loosely with a kitchen towel, and let them proof at room temperature for 1 hour. Transfer the baking sheet to the refrigerator for another hour to chill.

Preheat the oven to 400°F (200°C). Prepare an egg wash by whisking together an egg and a splash of milk. Brush the croissants gently with the egg wash. Bake for 20 minutes until golden and flaky.