Preheat the oven to 325° F. Grease and lightly flour a 12-cup tube pan.
Using a stand mixer, cream the butter until smooth, about 2 minutes. Add the sugar, and beat until fluffy, about 5 minutes. Add the grapefruit zest, grapefruit juice, and vanilla extract and mix until well combined.
Add the eggs to the batter, one at time, mixing each one until it’s well incorporated. Slowly pour in the sifted flour and mix until the batter is smooth and fluffy. Stir together the sour cream, salt, and baking soda, and then add to the batter, mixing until well incorporated. Pour the batter into the pan and bake for an hour and 30 minutes, or until an inserted knife pulls out clean.
Cool the cake for 15 minutes and then invert onto a platter. Sprinkle with powdered sugar and serve warm.
Notes
This recipe can be divided in half and baked in a loaf pan. You can also flavor this with lemon, orange, or lime juice, using the same amounts. For a bit of crunch, you can add to the batter 1/2 cup chopped pecans or 1/4 cup poppy seeds.