In a stand mixer, combine all-purpose flour, granulated sugar, salt, and instant yeast. Gradually pour in cold whole milk, mixing on low speed until combined. Knead the mixture for about 5 minutes until the dough is slightly tacky but smooth. Form into a ball, wrap in plastic, and chill in the refrigerator for 30 minutes.

Beat softened unsalted butter with a bit of flour until smooth. Shape into a rectangle, about 1/2 inch thick, wrap in parchment paper, and refrigerate for another 30 minutes.

Roll the chilled dough out into a rectangle, about 1/4 inch thick. Place the butter layer in the center, fold edges over to encase it, seal tightly, then roll into another rectangle, about 1/4 inch thick. Fold into thirds, wrap in plastic, and refrigerate for 30 minutes.

Repeat the folding and chilling process two more times. After the final fold, chill in the refrigerator for at least 4 hours or overnight.

Roll the dough into a large rectangle, about 1/4 inch thick. Cut into long triangles, each about 5 inches wide. Place a piece of chocolate at the wider end, roll tightly, and curve the ends.

Cover the croissants with a towel and let proof at room temperature for about 1 hour. For best results, refrigerate for an additional hour while preheating the oven.

Whisk together a large egg and a splash of milk for the egg wash, then brush over the croissants. Bake in the preheated oven at 400°F (204°C) for about 20 minutes.