STEP 1
Make the coating: Combine the cacao nibs, Demerara sugar and salt in a food processor and pulse for several seconds at a time until the mixture is the consistency of very coarse sand. Transfer the mixture to a small bowl and set aside. (Keep the food processor out for the next step; there’s no need to wash it.)
STEP 2
Make the dough: Add the flour, powdered sugar, cocoa powder and salt to the food processor and pulse a couple of times to combine. Add the butter pieces and process in long pulses until the butter breaks down into very fine bits. In a small bowl, whisk together the egg, sour cream and vanilla until smooth, then remove the lid and sprinkle the mixture over the flour mixture. Replace the lid and process until a smooth dough forms a ball around the blade.
STEP 3
Chill the dough: Scrape the dough onto a clean work surface, then cut into 3 equal pieces (each portion should be about 230 grams). Roll each portion into a 9-inch-long log, then place each log on a long piece of plastic wrap. With your hands, flatten and press each portion into an 11-by-3-inch rectangle. Wrap in the plastic, taking care to form squared-off packets with no (or few) air pockets, then roll the packets with a rolling pin to create neat rectangles of even thickness. Transfer the packets to the refrigerator in a single layer and chill for at least 1 hour (and up to 2 days).
STEP 4
Make the filling: Bring an inch of water to a simmer in a small saucepan over medium heat. Combine the chocolate and butter in a medium heatproof bowl and set over the saucepan. Reduce the heat to low and stir the chocolate mixture occasionally until completely melted. Remove the bowl from the saucepan (carefully, as it’s hot) and let cool for a minute, then add the powdered sugar, flour, egg yolk and vanilla, and whisk until completely smooth. Set aside, stirring occasionally until the mixture is cooled and the consistency of loose peanut butter.
STEP 5
Roll out the dough and fill: Remove a piece of dough from the refrigerator, unwrap and place on a long piece of lightly floured parchment paper. Dust the dough’s surface with flour and roll into a thin 15-by-6-inch rectangle, lifting the dough once or twice and adding flour as needed to prevent sticking. Use a knife or wheel cutter to trim and straighten the edges (discard scraps). Scrape about a third of the melted chocolate mixture (about ⅓ cup) onto the dough and, with a small offset spatula, spread it all the way to the edges, leaving a ½-inch wide strip on one long side. (If the chocolate mixture has cooled to the point that it’s difficult to spread, place it back on the saucepan over low heat and stir just until spreadable again.)
STEP 6
Starting at the long side with full chocolate coverage, use the parchment to help you roll the dough away from you into a long, tightly coiled log, dusting away any excess flour. Transfer the log to a baking sheet seam-side down and set aside, then repeat the rolling and filling process with the remaining two pieces of dough, the same piece of parchment and the remaining chocolate mixture, dividing evenly. Place the logs on the same baking sheet, transfer to the refrigerator, and chill until firm, 20 to 25 minutes.
STEP 7
Heat the oven and bake: As the dough chills, arrange an oven rack in the center position and heat to 350 degrees. Line a separate large rimmed baking sheet with
parchment paper and set aside.
STEP 8
Coat the cookies: Beat the remaining egg white in a small bowl with a fork just until liquidy. Remove the unlined baking sheet from the refrigerator and transfer the logs to the work surface. Sprinkle the cacao nib mixture across the unlined baking sheet in an even layer, then use a pastry brush to brush a thin layer of the egg white across the surfaces of the logs, coating them completely. Place them back on the baking sheet and roll the logs in the cacao nib mixture, rotating and pressing gently, until they’re coated all over in an even layer with no bare spots. Transfer the baking sheet to the freezer and chill until the dough is very firm, another 10 to 15 minutes.
STEP 9
Slice and bake the cookies: Transfer one log at a time to a cutting board and use a sharp knife to trim away the ends, revealing the spiral (discard the ends). Cut the log in half crosswise, then cut each half crosswise into thirds so you have 6 equal segments. Cut each segment in half crosswise, but this time, angle the knife at 45 degrees. Arrange the pieces on the prepared baking sheet, resting on their long sides.
STEP 10
Repeat the slicing process with the remaining logs and place on the same baking sheet, spacing them evenly. (It will be a tight fit; the cookies will slump a bit but not spread much.) Transfer the baking sheet to the oven and bake until the cookies are puffed and firm to the touch, 20 to 25 minutes. Let them cool completely on the baking sheet. Dust the smooth portions with powdered sugar, leaving the spirals exposed, before serving.
TIP
The dough can be chilled for up to 2 days. The baked and cooled cookies can be stored in an airtight container at room temperature for several days. They will soften a bit as they sit, and the powdered sugar will eventually disappear.
