In a stand mixer, combine all-purpose flour, granulated sugar, salt, and instant yeast. Gradually pour in cold whole milk and mix on medium speed for about 5 minutes, until soft and slightly tacky. Cover with plastic wrap and refrigerate for 30 minutes.

Prepare the butter layer by beating softened unsalted butter with all-purpose flour until smooth. Shape into a rectangle, wrap in plastic wrap, and refrigerate for another 30 minutes.

Roll the chilled dough into a rectangle (12x18 inches). Place the chilled butter layer in the center and fold edges over to encase the butter completely. Roll into a larger rectangle (12x24 inches) and fold into thirds. Refrigerate for 30 minutes.

Repeat the rolling and folding process (total of three times) while keeping ingredients cold.

After the final fold, cover and refrigerate for at least 4 hours or overnight.

Roll out dough into a large rectangle (12x36 inches). Cut into rectangles (4x8 inches) and place chocolate batons on the wider end, rolling it around the chocolate to form croissants.

Arrange croissants on lined baking sheets, spacing them apart. Cover loosely with a kitchen towel and let proof at room temperature for about 1 hour. Refrigerate for another hour.

Preheat oven to 400°F (204°C). Prepare egg wash by mixing egg with milk, then brush over each croissant. Bake for 20 minutes until puffed and golden brown.

Remove from oven and cool on wire racks. Dust with confectioners' sugar before serving.