Cut grapefruit in half. Using grapefruit knife, cut sections and loosen fruit from shell. Dot each half with 1 teaspoon of butter. Sprinkle each half with 1 tablespoon of brown sugar. Sprinkle 1/2 teaspoon of cinnamon over brown sugar.
Place in baking pan and bake in preheated oven at 350 degrees for 5 minutes, then broil until top is bubbly. Remove from oven and place in serving bowl and put a maraschino cherry in the center.