Slice top from rolls and place bottoms in baking pan. Layer ham and cheese on rolls. Replace tops. Combine butter,poppy seeds,mustard,Worcestershire, and onion powder and mix well. Drizzle over rolls. Refrigerate until butter is firm or overnight. Cover with foil and bake at 350° for 30 minutes or until cheese melts. Uncover and cook for 2 additional minutes. Serve warm or cold.