These rich, yet airy flourless chocolate cookies have crisp edges and chewy interiors. They rely on whipped egg whites for their structure. To ensure your whites attain the proper volume with beating, make sure the mixer bowl, whisk and the whisk attachment are perfectly clean and without any trace of grease or fat. Either Dutch-processed or natural cocoa works well in this recipe. Leftover cookies can be stored in an airtight container for up to three days; the edges will lose their crispness but the cookies will still taste good.