Choux au Craquelin: Cream Puffs With Crispy Cookie Tops

To Make the Craquelin Top by Hand: In a medium bowl and using a flexible spatula, mash together brown sugar and butter, then stir until smooth and creamy, about 3 minutes. Add flour and salt, thoroughly mixing until no dry flour remains and a damp, crumbly meal has formed, about 2 minutes. Alternatively, to Make the Craquelin Top in a Stand Mixer: In the bowl of a stand mixer fitted with the paddle attachment, beat the brown sugar and butter on medium speed until smooth and creamy, about 2 minutes. Scrape down the bowl and beater with a flexible spatula. Add flour and salt, then beat on medium-low speed until no dry flour remains and a damp, crumbly meal has formed, about 2 minutes. Using your hands, bring dough together to form a ball (it will be a bit sticky but do not add extra flour). Place a sheet of parchment paper on a work surface and set dough on top. Shape dough into a roughly 6- by 8-inch flat rectangle. Cover with a second sheet of parchment paper. Using a rolling pin, roll out dough into a 1/8-inch-thick rough rectangle 1/8 inch thick (it should be about 12-by-14 inches in size); reposition both sheets of parchment paper as needed to minimize creasing. Transfer dough, along with parchment paper, to a rimmed baking sheet and freeze until cold, about 5 minutes (or refrigerate until cold, about 15 minutes). Meanwhile, line a second baking sheet with parchment paper. Remove top sheet of parchment paper. Using a 2-inch round cutter, stamp out 18 pieces of craquelin. Working quickly, transfer craquelin rounds to the prepared baking sheet in a single layer, and return to freezer; you may need to slide an offset spatula under the cut-outs to loosen them from the parchment paper. If desired, craquelin dough scraps can be re-rolled out, chilled, and cut into new rounds (see Make Ahead section for more on holding extra, uncooked rounds). Adjust oven racks so that one is in upper-middle position, the other is in lower-middle position, and preheat oven to 375°F (191°C). Line two aluminum baking sheets with parchment paper. Pipe a small amount of choux paste under each corner of parchment paper (the dough acts as a glue and keeps the paper in place as you pipe). Holding the filled pastry bag at a 90° angle, apply steady downward pressure and pipe a 2-inch-wide choux (see note). To stop piping, cease applying pressure and swirl pastry tip away. Continue to pipe choux about 3 inches apart, for a total of 9 choux. Repeat with second tray. Top each choux with a craquelin cut-out, making sure it’s positioned parallel to the floor of the baking sheet, then press down slightly to ensure it adheres to the highest point of the piped choux. Bake both trays, switching racks and rotating trays front to back after 20 minutes, until choux au craquelin are puffed, deeply golden brown, and feel hollow when lifted, about 30 minutes total. For sandwich-style choux au craquelin, proceed to the next step; for piped-in choux au craquelin, proceed to step 11. To Fill Sandwich-Style Choux au Craquelin: Turn off oven. Let trays rest in oven with oven door partially open for 30 minutes. Remove from oven and let cool completely, about 15 minutes. Using a serrated knife, slice each puff in half to create a top and bottom "bun.” Transfer your choice of filling (see notes section below for filling instructions) to a pastry bag fitted with a 3/4-inch star tip. Working with one at a time, begin piping with steady pressure to generously fill the bottom half of each choux. Cover with the top half of each choux. Repeat until all choux au craquelin are filled. Serve immediately. To Fill Piped-In Choux au Craquelin: Working quickly, while choux au craquelin are still hot, gently insert the tip of a paring knife into the underside of each choux and rotate in a circular motion to create a small hole about 1/4 inch in size, then return to tray. Set both trays in the turned-off but still warm oven with the door partially open for 30 minutes. Remove from oven and let cool completely, about 15 minutes. Working with one at a time, insert tip of pastry cream–filled bag into hole in each choux and begin piping with steady pressure until filled (you can tell because the choux will feel heavy and pastry cream will start to overflow the hole). Wipe away any excess pastry cream. Repeat until all choux au craquelin are filled. Serve immediately.