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. Sprinkle batter evenly with almonds, if desired.
Bake at 400° for 8 minutes. Remove from pans; cool on wire racks. Sprinkle with powdered sugar, if desired.