In a stand mixer, combine 4 cups of all-purpose flour, 1/4 cup of granulated sugar, 2 teaspoons of salt, and 1 packet of instant yeast. Slowly pour in 1 1/2 cups of cold whole milk and mix until combined. Knead the dough for about 5 minutes on medium speed until it is soft yet slightly tacky. Cover the dough and refrigerate for 30 minutes to allow it to rest.

While the dough rests, make the butter layer by mixing 1 cup of softened unsalted butter with 1/2 cup of all-purpose flour in a bowl. Beat together until smooth, then shape into a rectangle approximately 8x6 inches. Wrap in plastic wrap and refrigerate for another 30 minutes.

Roll out the chilled dough on a lightly floured surface into a large rectangle, about 16x12 inches. Place the chilled butter rectangle in the center of the dough. Fold the dough over the butter, sealing the edges tightly to encase the butter completely.

With a rolling pin, gently roll the encased dough into a long rectangle, about 24x10 inches. Fold the dough into thirds, like a letter, and then wrap it in plastic wrap. Refrigerate for 30 minutes.

Roll the dough out again into a 24x10-inch rectangle, then fold into thirds once more. Repeat this process two additional times, chilling the dough in the refrigerator for 30 minutes between each rolling.

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

Once chilled, roll the final dough out into a large rectangle, about 24x12 inches, and cut it into triangles. Place chocolate batons or semi-sweet chocolate bars at the thick end of each triangle and roll them up toward the point, tucking the ends slightly.

Cover the shaped croissants with a clean kitchen towel and let them proof at room temperature for 1 hour. After the first hour, place them in the refrigerator for another hour.

Preheat your oven to 400°F (204°C). While your oven heats, prepare the egg wash by whisking 1 egg with a splash of milk. Brush the tops of the croissants with this mixture for a golden finish. Bake for 20 minutes until they are puffed and golden brown.

After baking, let the chocolate croissants cool on a wire rack for a few minutes. Dust with confectioners' sugar before serving.