Preheat oven to 350°. In a small bowl, combine the butter, mustard, sunflower seeds and sugar. Place bun bottoms, cut side up, in an ungreased 15x10x1-in. baking pan. Top each with ham and mozzarella. Brush with half the butter mixture.

Place bun tops on. Brush with remaining butter mixture. Bake, uncovered, until cheese is melted, 10-15 minutes.