Cook pasta shells according to package directions; drain. Return to pan; toss with butter. Cool until easy to handle.
Meanwhile, in a large skillet, cook ground beef until browned, stirring to break up ground beef; drain off fat. Stir in cream cheese and chili powder until combined. Remove from heat; cool slightly. Divide beef mixture evenly among the pasta shells.
Spread about 1/2 cup salsa in a 2-quart rectangular baking dish. Arrange filled shells in dish. Spoon remaining salsa over. Cover; bake in a 350 degrees F oven for 15 minutes. Sprinkle with cheese. Bake, uncovered, for 15 minutes more. Sprinkle with chopped tomato and sliced ripe olives, if desired. Makes 6 servings.