Preheat oven to 400 degrees F. Lightly grease 6 8-ounce ramekins or 6-ounce custard cups. Line a shallow baking pan with foil; arrange prepared dishes in pan; set aside. Peel and remove pith and seeds from 1 or 2 of the grapefruits and cut into 1/4-inch thick slices (you will need 6 slices). Place 1 grapefruit slice into the bottom of each prepared dish. Squeeze 1/2 cup of juice from the remaining grapefruit; set aside. In a small bowl stir together 1/3 cup sugar and melted butter until nearly smooth; spoon over slices in dishes.
In a large bowl combine flour, 1 cup sugar, baking powder, and salt; make a well in the center and set aside. In a medium bowl combine the reserved grapefruit juice, oil, eggs, and vanilla. Add juice mixture to flour mixture, stirring to combine. Divide batter among ramekins and bake for 25 to 30 minutes or until a toothpick inserted in the centers comes out clean. Let cool in ramekins for 10 minutes. Loosen sides and invert onto plates. Serve warm garnished with mint and powdered sugar, if desired.