In a medium bowl, combine 1 cup of all-purpose flour, 2 1/2 teaspoons of instant yeast, and 1 1/4 cups of warm milk. Stir the mixture until well combined and let it sit for 20 minutes until bubbly and frothy. Gradually mix in the remaining flour (2 1/2 cups), 1/4 cup of sugar, and 2 teaspoons of salt. Knead the dough for about 2 minutes until smooth, then refrigerate for at least 2 hours or overnight.

Take 9 ounces of cold butter and place it between two sheets of parchment paper. Use a rolling pin to shape the butter into a 7x7 inch slab. Chill the butter slab in the refrigerator for 10 minutes.

On a floured surface, roll the rested dough into a 10x10 inch square. Place the cold butter slab diagonally in the center of the dough. Fold each corner of the dough over the butter, pinching to seal it inside completely. Roll the dough into a 10x24 inch rectangle, maintaining an even thickness.

After the final fold, roll the dough out once more into a 10x24 inch rectangle. Cut the dough into long strips, about 4 inches wide. Take a piece of bittersweet chocolate and place it at one end of each strip. Roll the dough tightly around the chocolate, placing the rolled croissants seam-side down on a lined baking sheet.

Cover the baking sheet with a clean kitchen towel, allowing the rolled chocolate croissants to proof overnight in the refrigerator or at room temperature.

Preheat your oven to 400°F (200°C). Prepare an egg wash by whisking together 1 egg with 1 tablespoon of heavy cream, and gently brush it over the tops of the proofed croissants. Bake for 25 to 30 minutes, rotating halfway through.

Once the chocolate croissants are out of the oven, transfer them to a wire rack and allow them to cool for a few minutes.