To make the crackle tops: Remove the butter from the refrigerator 10 minutes before you're ready to make the dough.
After 10 minutes, cut the butter into small cubes and toss it and the brown sugar and salt into a food processor. Process until almost blended. Add the flour and pulse until the dough forms moist curds. Pulse in the vanilla.
Scrape the dough onto a piece of parchment or wax paper and shape it into a disk. Cover with another piece of parchment or wax paper and roll the dough out so that it's about 1/16th inch thick. Slide the dough, still between the paper, onto a cutting board and freeze it for at least 2 hours. (The dough can be frozen for up to 2 months, in which case it should be wrapped airtight.)
"Baking Chez Moi" by Dorie Greenspan (c) Houghton Mifflin Harcourt 2014. Provided courtesy of Dorie Greenspan. All rights reserved.