Preheat your oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with nonstick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium sized mixing bowl, whisk together the cake flour, baking powder, salt and poppy seeds. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and the sugar for two minutes on medium-high speed.
Scrape down the sides of the bowl and begin to add the egg whites on medium speed, scraping down the sides and bottom of the bowl between addition.
Add the sour cream and beat until incorporated, about one minute.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1/2 cup of the grapefruit juice. Repeat the additions, 1/3 cup flour mixture, 1/2 cup juice and the final 1/3 cup flour mixture. Add the vanilla. Only mix until the flour mixture is incorporated, about 30 seconds. Scrape down the sides and bottom of the bowl and mix again for another 20 to 30 seconds on low.
Pour about 18 ounces of cake batter into each of the three 8-inch cake pans or 12 ounces in each of the 6-inch pans and bake for 28-30 minutes - until a toothpick inserted into the center of the cake comes out with a few moist crumbs on it. Let cool in the pans for about 15 minutes and then invert onto a wire rack to cool completely. (Tip: spray your wire cooling racks with nonstick spray).