Position an oven rack in the lower third of the oven and heat the oven to 400°F. Line a heavy baking sheet with parchment. In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the chopped chocolate, tossing until the pieces are evenly distributed and coated with flour. Cut in the butter with a pastry blender or two table knives until the largest pieces of butter are about the size of peas.
In a small bowl, stir the cream and egg yolks just to blend. Add this all at once to the flour mixture. Stir with a fork to begin combining the wet and dry ingredients and then use your hands to gently knead the mixture together until all the dry ingredients are absorbed into the dough and it can be gathered into a moist, shaggy ball. Don’t overknead: This dough is sticky but benefits from minimal handling. Set the rough ball in the center of the prepared baking sheet and pat it gently into a round about 1 inch thick and 7 inches in diameter. Don’t be tempted to make the round any flatter.
With a sharp knife or a pastry scraper, cut the round into eight wedges; separate the wedges. Brush the scones with the egg-milk glaze (you won’t need to use all of it) and sprinkle with the sugar. Bake until the scones are deep golden and a toothpick inserted into the center of a wedge comes out clean, 18 to 22 minutes. Slide the parchment onto a rack and let the scones cool for 10 to 15 minutes before serving.
Photo: Scott Phillips