3 tbsp unsalted butter, melted
3 tbsp light brown sugar
1/2 tsp cinnamon
Preheat broiler for 10 minutes.
Cut each grapefruit in half and loosen each section.
Combine butter, sugar and cinnamon in a small bowl.
Spread 1/6th of the mixture over each cut side of the grapefruit.
Place on a 15" X 10" baking sheet, cut side up and broil for about 5 minutes, until top is bubbly.