Heat the broiler to high (or alternatively, use a kitchen torch).
Prepare the grapefruit by cutting the top and bottom rind to create a flat surface (removing about ¼ inch from the top and bottom of the grapefruit).
Cut the grapefruits in half and place them face down on a layer of paper towels to draw excess moisture from the surface (this will allow the sugar to adhere to the citrus during cooking). Allow them to sit for 5-10 minutes.
Line a sheet tray with foil.
Pour the sugar into a small plate and dip the tops of the grapefruit into the sugar until evenly coated.
Place the grapefruits on the tray and broil for 8 minutes or until caramelized (make sure to check them early as sugar will burn quickly). Alternatively, use a kitchen torch to caramelize the sugar, making small circular motions to evenly brown the surface.
Serve warm with a grapefruit knife or spoon.