Preheat oven to 350°. Whisk together flour, baking powder and salt. Combine next seven ingredients. Gradually stir flour mixture into yogurt mixture, then pour into a 9-in. round baking pan coated with cooking spray. Bake until a toothpick inserted in center of cake comes out clean, 25-30 minutes. Cool., For glaze, mix confectioners' sugar with enough grapefruit juice to reach desired consistency; drizzle glaze over top, allowing some to flow over sides. Top cake with grapefruit and mint if desired., Freeze option: Omit glaze. Securely wrap cooled cake in plastic and foil; freeze. To use, thaw at room temperature. Prepare glaze; top as directed.