In a medium-size frying pan over medium heat, melt butter.
Add onion and garlic; cook until onion is limp.
Add beef and cook until browned and crumbly; drain off fat.
Stir in catsup, chili powder, and olives.
If made ahead, cover and refrigerate; reheat when needed.
Arrange muffin halves, cut side up, on a shallow baking pan.
Evenly divide hot meat mixture among the twelve muffin halves; top with cheese.
Place in a preheated broiler about 6 inches from heat just until cheese melts.