Cut the grapefruits in half, and use a small serrated knife to cut out the sections in the halves. Spoon the sections and juice into a bowl, then scrape out all of the remaining thick skins and pulp. Spoon the sections from the bowl back into the halves. This is best done one half at a time.
Sprinkle sugar and cinnamon over the top of each grapefruit half, adjusting as you see fit. Dot with butter. Place the halves on a cookie sheet.
Broil for 3 to 5 minutes, until the butter is melted and sugar is starting to turn brown.