1) Preheat broiler on high.
2) Cut grapefruits in half and use a knife to cut out each segment away from the tough walls.
3) Mix butter, brown sugar, sugar, cinnamon and pinch of salt in a small bowl.
4) Top each grapefruit half with a good sprinkle of the topping.
5) Broil grapefruits on a baking sheet for about 4 minutes until the topping is caramelized.
6) Let cool briefly and serve immediately.