Preheat oven to 425 degrees F (220 degrees C). Remove and chop mushroom stems.
In a saucepan over medium heat, combine ground beef, onion, and garlic. Cook until beef is no longer pink; drain. Mix in chopped mushroom stems, 1 tablespoon butter, bread crumbs, salt, and pepper. Cook, stirring frequently, for 5 minutes. Remove from heat, and stir in cream.
Dip mushroom caps in 1/4 cup melted butter, and stuff generously with meat mixture. Arrange stuffed mushrooms in a baking dish. Sprinkle with chili powder.
