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.