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1
Into a small bowl, sift together the flour, cocoa, baking powder, and salt.
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2
In a stand mixer fitter with the paddle attachment (or in a bowl by hand), beat together the butter and the sugar on medium speed just until smooth.
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3
Add the vanilla, then beat in the egg and egg yolk.
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4
Gradually add the cocoa mixture to the butter mixture, mixing until completely incorporated and no streaks of butter remain.
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5
On a lightly floured work surface, divide the dough into quarters, and shape each quarter into a log about 7 inches (18 cm) long and 1 1/2 inches (4 cm) in diameter.
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6
Wrap the logs in plastic wrap and refrigerate until theyre firm enough to slice, about 1 hour.
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7
Position racks in the upper and lower thirds of the oven; preheat the oven to 350F (175C).
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8
Line 2 baking sheets with parchment paper or silicone baking mats.
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9
Slice the logs into disks 1/2 inch (1.5 cm) thick and place the disks on the prepared baking sheets, spaced about 1/2 inch (1.5 cm) apart.
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10
Bake, rotating the baking sheets midway through baking, until the cookies are puffed and slightly firm, 10 to 12 minutes.
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11
Remove from the oven and immediately sprinkle the cookies with a bit of sugar.
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12
Let the cookies cool on the baking sheets until firm enough to handle, then use a spatula to transfer them to a wire rack.
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13
They will continue to firm up and get snappy as they cool.
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14
The dough can be refrigerated for up to 5 days or frozen for up to 1 month.
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15
The baked cookies can be kept in an airtight container for 2 days.
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16
Instead of forming the dough into logs and slicing them, the dough can be rolled out and cut into shapes with cookie cutters.
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17
If you have coarse-crystal sugar, the sliced unbaked cookies can be dredged in them on one side, then baked sugar side up.
