Preheat the oven to 375°F (190°C) and place rack in center of oven.
In a small bowl whisk together heavy cream, egg, and vanilla extract.
In a large bowl, whisk together flour, cocoa powder, sugar, baking powder and salt.
Using a pastry blender or two knives, cut the butter into the flour mixture until it resembles coarse crumbs. Stir in the chocolate chips. Add the cream mixture and stir just until the dough comes together (add more cream and/or flour as necessary).
Transfer the dough to a lightly floured surface and knead a few times. Shape the dough into a 7-inch (18 cm) round and cut into eight wedges. Place them on the baking sheet.
Bake for about 20 minutes or until they are firm around the edges but a bit soft in the center. A toothpick inserted into the center of a scone will come out clean. Cool on a wire rack.
Mix the glaze ingredients together in a bowl. Drizzle on top of the scones (optional).