Preheat the oven to 350 degrees F. In a medium-sized bowl, whisk together 5 ounces flour, salt, and confectioners’ sugar.
Using a pastry blender, cut the butter into the flour mixture until it resembles coarse crumbs. Press the mixture into an un-greased 8x8 inch pan. Bake for 15 to 18 minutes, or just until golden.
While the crust is baking, prepare the filling. In a large bowl, use an electric mixer on medium speed to beat the eggs, sugar, grapefruit juice, lemon juice, and remaining flour for about 2 minutes or until smooth, scraping down the bowl periodically with a spatula. Stir in grapefruit peel.
Pour the filling over the hot crust, then bake for an additional 20 minutes, or until light brown around the edges and the center is set.
Transfer to a wire rack to cool. Cover and refrigerate for 1 to 2 hours. Dust lightly with powdered sugar, then cut into 9 bars.