Cocoa nibs are roasted cocoa beans, broken into small pieces; they add a bittersweet crunch. Look for them on the baking aisle at specialty stores or large supermarkets. Superfine sugar dissolves easily into the meringue for a supple texture. If you can't find superfine sugar, process granulated sugar in a blender for a minute or two. Store meringues in an airtight container for up to one day. Serve as a crowd-pleasing follow-up to the Spice-Rubbed Braised Beef.
Place cream of tartar and egg whites in a large bowl; beat with a mixer at high speed until foamy. Gradually add superfine sugar, 1 tablespoon at a time, beating until stiff peaks form. Add 2 tablespoons unsweetened cocoa, cocoa nibs, instant espresso granules, and salt; beat just until blended.
Drop batter by tablespoonfuls onto prepared baking sheet. Bake at 225° for 1 hour. Turn oven off (do not remove pan from oven); cool meringues in closed oven at least 8 hours or until crisp. Carefully remove meringues from paper. Sprinkle with 1 teaspoon unsweetened cocoa, if desired.