Position racks in the upper and lower thirds of the oven and preheat to 425 degrees F. Line 2 baking sheets with parchment paper.
Pulse the butter, 3 tablespoons sugar, the cocoa powder, breadcrumbs, egg yolk, cinnamon and flour in a food processor until smooth.
Spread the remaining 1 cup sugar on a clean surface. Unfold the puff pastry and set it on top of the sugar, pressing gently to coat. Flip and coat the other side, then roll out the sheet into a 13-inch square.
Drop mounds of the cocoa mixture onto the puff pastry and spread in a thin, even layer. Using your fingers, roll 1 side of the pastry into the center, then roll the opposite side into the center to meet it. Trim the short ends with a knife. (If the rolled-up dough is soft, refrigerate until firm.)
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. 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. Let cool 1 minute on the baking sheets, then transfer to racks to cool completely.
Photograph by Andrew Purcell
Courtesy of Food Network Magazine