Preheat oven to 375F. Lightly butter a baking sheet.
In a large bowl, stir together the flour, sugar, cocoa, espresso powder, cream of tartar, baking soda and salt.
Cut the butter into 1/2-inch cubes and distribute them over the flour mixture. With a pastry blender or two knives used scissors fashion, cut in the butter until the mixture resembles coarse crumbs. In a small bowl, stir together the buttermilk, egg, and vanilla. Add the milk mixture to the flour mixture and stir to combine. The dough will be very sticky. Stir in the chocolate until evenly distributed.
Using a 1/3-cup measuring cup, drop the dough onto the prepared baking sheet, leaving about 4 inches between scones. Bake for 25 to 30 minutes, or until a cake tester or toothpick inserted into the center of a scone comes out clean. Remove the baking sheet to a wire rack and cool for 5 minutes. Using a spatula, transfer the scones to the wire rack to cool. Serve warm or cool completely and store in an airtight container.