In a large bowl stir together flour, cocoa powder, brown sugar, baking powder, baking soda, and salt. Using a pastry blender, cut in the 1/2 cup butter or margarine until mixture resembles coarse crumbs. Make a well in the center of dry ingredients; set aside.
Combine egg yolk and yogurt; add to dry ingredients. Add chocolate pieces. Stir mixture until moistened. On a lightly floured surface, gently knead dough 10 to 12 strokes or until dough is nearly smooth. Roll or pat dough into a 9-inch circle; cut in 10 wedges. Place wedges 1 inch apart on an ungreased baking sheet.
Bake in a 375 degree F oven about 18 minutes or until bottoms are lightly browned. Remove from baking sheet; cool on a wire rack for 5 minutes.
In a small mixing bowl stir together the 1 cup powdered sugar, 1 tablespoon butter or margarine, milk, and vanilla. Add more milk, 1/4 teaspoon at a time, until glaze is of a drizzling consistency. Drizzle over scones; if desired, dust top with additional powdered sugar. Serve warm. Makes 10 scones.