Preparation Preheat oven to 350 degrees. Grease and flour a standard (12-cup) Bundt pan; set aside. In a small bowl, combine lemon, orange and grapefruit zests. Reserve two teaspoons zest to use in the glaze. Add sugar to the remaining zest; mix until zest is incorporated and sugar is the texture of wet sand. In the bowl of an electric mixer fitted with the paddle attachment, on medium speed, combine the zested sugar and eggs, beating until creamy. Add baking powder, vanilla and oil. Beat until smooth. Add half the flour, followed by the citrus juice, followed by the remainder of the flour. Beat until smooth. Pour batter into prepared pan. Bake for about 45 minutes, until a tester inserted into the center comes out clean. Allow cake to cool for 10-15 minutes before removing from pan. Set aside to cool completely before glazing. Prepare the glaze: Combine all ingredients in a small bowl. Drizzle over cooled cake.