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1
Line a baking sheet with parchment; set aside.
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2
In a large bowl, whisk together the flour, cocoa, granulated sugar, baking powder, baking soda, and salt.
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3
Using a pastry blender, cut in the butter until the mixture resembles coarse crumbs with a few larger clumps remaining.
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4
Fold in the chocolate.
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5
Whisk together the whole egg and 1/2 cup plus 1 tablespoon cream.
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6
Add egg mixture to the flour mixture; using a rubber spatula, fold in, working in all directions and incorporating crumbs at the bottom of the bowl, until dough just comes together.
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7
Turn out the dough onto a lightly floured work surface, and gently pat into a 18-by-3-inch rectangle about 1 inch thick.
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8
Using a sharp knife or a pastry wheel, cut the rectangle into six 3-inch squares.
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9
Place squares about 2 inches apart on the prepared baking sheet.
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10
Cover with plastic wrap and freeze until the dough is very firm, at least 1 hour or overnight.
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11
(At this point, you can freeze the unbaked scones in a resealable plastic bag until ready to bake, up to 3 weeks.)
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12
Preheat the oven to 375F.
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13
In a small bowl, lightly beat the egg yolk with the remaining tablespoon heavy cream; brush over the tops of the scones and sprinkle generously with sanding sugar, if using.
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14
Bake, rotating the sheet halfway through, until the sugar on top of the scones turns golden all over, or a cake tester inserted in the center of a scone comes out clean, 25 to 30 minutes.
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15
Transfer to a wire rack to cool.
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16
They are best eaten the day they are baked.
