Prepare the Dough: In a stand mixer, combine all-purpose flour, sugar, salt, and yeast. Slowly add cold milk and knead on low speed for about 5 minutes until smooth. Shape into a ball, wrap in plastic, and refrigerate for at least 30 minutes.

Make the Butter Layer: Beat softened butter with a spoonful of flour until smooth. Shape into a 6-inch rectangle, wrap in parchment, and refrigerate.

Laminate the Dough: Roll out chilled dough to a rectangle, place chilled butter layer in center. Fold edges to encase and seal tightly. Roll out to an 18-inch rectangle, perform the first fold and refrigerate for 30 minutes.

Chill the Final Dough: Repeat rolling and folding twice more, refrigerating each time for 30 minutes. Finish with a final fold and refrigerate for at least 4 hours or overnight.

Shape the Croissants: Roll dough into a 24-inch rectangle, cut into triangles. Place chocolate at the wide end, roll tightly, and pinch ends.

Proof the Croissants: Arrange on baking sheets with space in between. Cover and proof at room temperature for about 1 hour. Refrigerate for an additional hour.

Bake the Croissants: Preheat oven to 400°F (200°C). Create an egg wash with egg and milk, brush over croissants. Bake for 20 minutes, rotating halfway.

Cool & Serve: Let cool on a wire rack for a few minutes. Dust with confectioners’ sugar if desired. Serve warm.