In a stand mixer, combine 4 cups of all-purpose flour, 1/4 cup of granulated sugar, 1 teaspoon of salt, and 2 teaspoons of instant yeast. Gradually add 1 cup of cold whole milk while mixing on low speed for about 5 minutes, until the dough is smooth and elastic. Shape it into a ball, cover it with plastic wrap, and refrigerate for 30 minutes.

While the dough chills, prepare the butter layer. Soften 1 cup of unsalted butter, mixing it with 1/4 cup of all-purpose flour until it forms a smooth, pliable mixture. Shape the butter into a rectangle about 1/2 inch thick and wrap it in plastic wrap. Place it in the refrigerator for another 30 minutes.

After the chilling, roll the dough out on a floured surface into a rectangle approximately 12x18 inches. Place the chilled butter layer in the center and fold the dough over to completely encase the butter, pinching the edges to seal.

Using a rolling pin, gently roll the dough into a larger rectangle (roughly 12x24 inches). Fold it into thirds, like a letter. Wrap it in plastic wrap and chill in the refrigerator for 30 minutes. Repeat this rolling and folding process two more times, chilling in between each fold.

After the final fold, refrigerate the dough for 4 hours or overnight. Roll the dough into a larger rectangle (about 12x36 inches). Cut the dough into rectangles of approximately 4x6 inches. Place a chocolate baton on the edge of each rectangle, then roll it tightly from the edge inward.

Arrange the rolled croissants on a baking sheet lined with parchment paper, leaving space between each one. Cover them loosely with a kitchen towel and allow them to proof at room temperature for about 1 hour. Refrigerate them for another hour for enhanced texture.

Preheat your oven to 400°F (204°C). In a small bowl, whisk together 1 egg and 1 tablespoon of milk to create an egg wash. Brush the tops of your croissants lightly with this mixture. Bake the croissants for about 20 minutes, until golden brown. Allow to cool slightly before serving.