Preheat oven to 350°F. Grease three 6-inch round cake pans and line with parchment paper. Set aside.
In a medium bowl, whisk together sugar, grapefruit zest, and ginger. Set aside.
The bowl of a stand mixer fitted with a paddle attachment (or using an electric hand mixer), beat together egg yolks and oil until slightly thickened. Add sugar mixture. Mix until well blended.
Add sour cream and grapefruit juice. Mix until fully incorporated.
In a large bowl, whisk together flour, baking powder, and salt. Add flour mixture in two additions to mixing bowl. Use a large spatula to gently fold flour into wet ingredients.
In separate clean bowl, use an electric hand mixer to beat egg whites until foamy. Add cream of tartar and continue to beat to soft peaks. Add whipped egg whites to mixing bowl in three additions. Use the large spatula to gently fold and incorporate whites into batter. The batter will loosen as the egg whites are added. Be careful not to overmix as the egg whites will deflate. Mix just until there are no longer and streaks of white.
Divide batter among prepared cake pans (about 300 grams batter per pan). Level batter as needed. Bake for 28-30 minutes until toothpick inserted in center of cake comes out clean, and cake starts to pull away from the sides of the pan.
Allow cake to cool in pan for 10 minutes. Run mini offset spatula or butterknife around perimeter of cake to ensure easily unmolding. Invert cake and transfer to wire rack to cool to room temperature.
Cake layers must be completely cool before frosting. NOTE: Cake can be baked up to 2 days before frosting. Wrap cooled cake in plastic wrap at room temperature until ready to assemble.