Heat oven to 400°F. Line a 24-cup mini muffin pan with paper liners.
In a large bowl, sift together the sugar, flour, cocoa, baking powder, and salt.
In a medium bowl, whisk the egg whites until foamy, then whisk in the oil and vanilla. Add the egg white mixture to the flour mixture, stirring just until moistened. Fold in the chocolate.
Divide the batter among the lined muffin cups and bake until a wooden pick inserted into the center comes out clean, 7 to 9 minutes. Remove from pans and let cool on wire racks. Decorate as desired.