Blend basil, parmesan cheese, olive oil and garlic in food processor or blender until well combined. Add yogurt, and process until smooth. Season with salt and pepper.
Preheat broiler. Place muffins on baking sheet. Broil 1 to 2 minutes, or until lightly toasted. Spread each half with basil mixture, and top with some tomato slices. Broil 1 to 2 minutes or until heated through.