Make Ahead and Storage: The cookies can be stored airtight for up to 1 week. Why It Works: Like brownies, these cookies get the bulk of their structure from eggs rather than flour. A healthy dose of cocoa powder doesn't provide structure in the same way that flour does, but it helps the batter more closely resemble cookie dough and not spread too much in the oven. Pro Tip: Err on the side of underbaking these cookies--they're loaded with chocolate, and just like melted chocolate, they up as they cool. You can think of baked goods made with substantial amounts of chocolates as a little like steaks: You need to allow for carryover cooking.