In a stand mixer, combine all-purpose flour, granulated sugar, salt, and instant yeast. Gradually add cold whole milk while mixing until the dough starts to come together, about 5 minutes. The dough should feel slightly tacky but smooth. Form it into a ball, wrap it in plastic wrap, and refrigerate for 30 minutes to relax the gluten.

While the dough chills, prepare the butter layer. Beat cold unsalted butter and all-purpose flour together in a bowl until it becomes a smooth cream. Shape the butter mixture into a rectangle, about 1/2 inch thick, and wrap it in plastic. Chill for 30 minutes in the refrigerator, ensuring it remains solid and cold for easy handling.

Roll the chilled dough on a lightly floured surface into a rectangle about twice the size of the butter square. Place the cold butter layer in the center of the dough, folding the edges over to encase it completely. Pinch the seams tightly to seal, then roll the dough into a larger rectangle, approximately 1/4 inch thick, sealing the edges as you go.

Now, perform the first fold by bringing the shorter edges of the rectangle toward the center, overlapping them slightly. This creates three layers of dough and butter. Wrap the folded dough in plastic wrap and chill in the refrigerator for 30 minutes before repeating the rolling and folding process two more times to build the flaky layers.

After completing the final fold, wrap the dough tightly in plastic and let it rest in the refrigerator for at least 4 hours, preferably overnight. This resting period allows the gluten to relax, which is essential for achieving perfectly flaky Homemade Chocolate Croissants.

Once rested, roll the dough out into a large rectangle again, about 1/4 inch thick. Cut the dough into long triangles, approximately 5 inches wide at the base. Place a chocolate baton at the wide end of each triangle and roll it up tightly toward the point. Ensure the pointed end is tucked underneath to maintain the shape.

Place the shaped croissants on baking sheets lined with parchment paper, ensuring they are spaced apart. Cover loosely with plastic wrap and allow them to proof at room temperature until doubled in size, about 1-2 hours, or refrigerate for a longer, cold fermentation.

Preheat your oven to 400°F (200°C). Prepare an egg wash by whisking together the large egg and a splash of milk. Brush this mixture gently over the surface of the croissants for a beautiful, golden finish. Bake the croissants in the preheated oven for 15-20 minutes, or until they are puffed up and golden brown.

Once baked, transfer the croissants to a wire rack to cool for a few minutes. Dust with confectioners' sugar if desired for an extra touch of sweetness. Allow them to cool slightly before serving to enjoy the gooey chocolate center.