Preheat oven to 350°F. Grease a 9″ loaf pan with butter, line with parchment and grease the parchment with more butter.
In a small bowl, whisk together the flour, baking powder and salt, set aside. In a large bowl, whisk together the sugar and olive oil.
Add the zest, eggs, grapefruit juice and vanilla, mix until well incorporated. Fold the flour in just until there are no streaks of flour, but be careful not to over mix or the cake will be tough. Pour the batter into the prepared pan. The batter should come up no higher than 2/3 of the way up the pan.
Bake grapefruit pound cake for about 1 hour, or until a tester comes out clean. Try not to over bake the cake. Let the cake cool for about 10 minutes.