In a stand mixer, combine the all-purpose flour, granulated sugar, salt, and instant yeast. Gradually add the cold whole milk while kneading until a soft dough forms, about 5 minutes. Shape into a ball, wrap in plastic wrap, and chill for 30 minutes.

Beat softened unsalted butter with all-purpose flour until smooth and creamy, forming a thick paste. Shape into a rectangle approximately 6x8 inches, wrap in parchment paper or plastic wrap, and refrigerate for another 30 minutes until firm.

Roll the chilled dough into a larger rectangle about 12x18 inches. Place the cold butter layer in the center, fold the edges of the dough over the butter, sealing it completely. Flatten gently with a rolling pin, then chill for 10 minutes.

Roll out the dough into a longer rectangle, approximately 12x24 inches. Perform a letter fold by folding one-third over the middle, then fold the remaining third over that. Wrap in plastic wrap and refrigerate for 30 minutes. Repeat rolling and folding two more times, chilling in between.

Wrap the laminated dough in plastic wrap and refrigerate for at least 4 hours or overnight to set the layers.

Roll the chilled dough into a long rectangle about 12x24 inches, cut into triangles. Place a chocolate baton at the base of each triangle and roll them up tightly, tucking the point underneath. Arrange croissants on a lined baking sheet.

Cover the croissants loosely with plastic wrap or a kitchen towel. Allow them to proof at room temperature for 1 hour until puffy, then refrigerate for an additional hour.

Preheat oven to 400°F (204°C). Prepare an egg wash by whisking the egg with milk. Brush the tops of the croissants and bake for 20 minutes or until golden brown and flaky. Cool slightly on a wire rack before serving.