Cocoa nibs, also called "cacao nibs," give these cookies a mottled look that resembles stone. We made the recipe with both chocolate-covered nibs made by Scharffen Berger and plain nibs from Dagoba Organic Chocolate, and both worked well. Find both types in gourmet markets. Or substitute 2 tbsp. finely chopped bittersweet chocolate if you prefer. Make up to 3 days ahead; store airtight.