Lightly spoon flour into a dry measuring cup; level with a knife.
Combine flour, sugar, and cocoa in a medium bowl, stirring with a whisk.
Whisk egg whites until foamy in a separate bowl.
Add oil and liqueur, if desired, stirring with a whisk.
Add egg white mixture to flour mixture, stirring just until moistened. Fold in cocoa nibs. Spoon batter evenly into 24 miniature muffin cups coated with cooking spray.