Combine 4 cups of all-purpose flour, 1/4 cup of granulated sugar, 1 teaspoon of salt, and 2 1/4 teaspoons of instant yeast in a stand mixer. Gradually add 1 1/4 cups of cold whole milk and mix until a soft, slightly tacky dough forms.

Knead the dough for about 5 minutes, then wrap it in plastic wrap and refrigerate for 30 minutes.

Prepare the butter layer by beating 1 cup of softened unsalted butter with 2 tablespoons of all-purpose flour until smooth, then shape into a rectangle and chill.

Roll out the chilled dough into a 14x10-inch rectangle, place the butter layer in the center and fold to encase.

Roll the dough into a 10x20-inch rectangle, fold into thirds, wrap in plastic and chill for 30 minutes.

Remove the dough, roll out again to 10x20-inch, and repeat folding process. Chill for another 30 minutes, twice more.

Chill the final fold for at least 4 hours or overnight in the refrigerator.

Roll out the dough into an 8x20-inch rectangle, cut into sixteen 2x5-inch rectangles, and shape into croissants with chocolate filling.

Proof croissants at room temperature for 1 hour, then refrigerate for 1 hour.

Preheat oven to 400°F (204°C). Brush with egg wash and bake for 20 minutes until golden brown.

Cool slightly on a wire rack before serving.