Put the flour, cocoa powder, sugar, baking powder and salt in a large bowl. Whisk to combine.
Add the cold butter, and using either a pastry blender or your hands, cut the butter into the flour until it is mostly mixed in but you have a few pea sized pieces left. Add the chopped chocolate
Beat the buttermilk, egg and vanilla in a liquid measuring cup and pour into the dry mixture. Stir until well combined, but be careful not to over mix the dough (you should still be able to see pieces of butter in the dough).
Sprinkle a tablespoon or so of flour on a dry, clean surface and lay the dough on top. Pat it down until you have a rectangle, about 3/4 inch thick. Using a biscuit cutter, cut the dough into biscuits and lay them on a parchment lined baking sheet.
Brush the biscuits with buttermilk and sprinkle with sugar. Bake for 12-15 minutes until the tops are dry and firm.
Serve warm with jam or butter. If you have any left, store them in an airtight container.
Notes
Storing leftovers: Store leftovers in an airtight container for up to 3 days.
Variations: You can add dried cranberries or orange zest to add flavor.