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1
Position racks in the upper and lower thirds of the oven and preheat to 425 degrees F. Line 2 baking sheets with parchment paper.
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2
Pulse the butter, 3 tablespoons sugar, the cocoa powder, breadcrumbs, egg yolk, cinnamon and flour in a food processor until smooth.
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3
Spread the remaining 1 cup sugar on a clean surface.
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4
Unfold the puff pastry and set it on top of the sugar, pressing gently to coat.
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5
Flip and coat the other side, then roll out the sheet into a 13-inch square.
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6
Drop mounds of the cocoa mixture onto the puff pastry and spread in a thin, even layer.
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7
Using your fingers, roll 1 side of the pastry into the center, then roll the opposite side into the center to meet it.
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8
Trim the short ends with a knife.
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9
(If the rolled-up dough is soft, refrigerate until firm.)
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10
Cut the dough roll crosswise into 1/2-inch-thick pieces and arrange cut-side down, about 2 inches apart, on the prepared baking sheets.
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11
Bake until golden and crisp on the bottom, 4 to 6 minutes; flip with a thin spatula, switch the position of the pans and continue baking until golden on the other side, 4 to 6 more minutes.
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12
Let cool 1 minute on the baking sheets, then transfer to racks to cool completely.
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13
Photograph by Andrew Purcell
