Pound pieces of steak between pieces of waxed paper to 1/2" thickness. In a large skillet, heat butter & saute sirloins for 2 minutes. Turn & season with salt & pepper. Cook to desired degree of doneness (2 minutes each side for rare). Pour brandy over steaks; stand back & using a fireplace match (or something with a long handle) ignite. When the flames die, transfer steaks to warm serving platter.
Add whipping cream, mustard, sour cream & Worcestershire sauce to juices in pan. Cook & stir until heated through. Pour sauce over steaks & serve.