Place oven rack in lower-middle position and preheat oven to 375 degrees Fahrenheit.
Mix flour, cocoa powder, baking soda, baking powder and salt in a medium mixing bowl with a whisk. This ensures the baking soda and baking powder are well distributed.
Using a pastry blender or two knives, cut in the butter until it’s in pea-sized pieces. Gently, stir in the buttermilk with a rubber spatula, until mostly combined and then stir in the chopped chocolate pieces.
Dump rough dough out onto lightly-floured surface and form into a square, about 6 inches by 6 inches and 1 ½ inches thick.
Using a serrated knife, score the top to create a grid of 9 shortcakes, “drawing” two lines across length wise and two across width wise.
Cut your shortcakes with the serrated knife and place onto a parchment-lined baking sheet.
Using a pastry brush, brush the shortcakes with the milk and sprinkle with coarse sugar.
Bake for 12-17 minutes, until they’re firm but not burned on the bottom. Cool completely.