Preheat oven to 350 degrees F. Spray a loaf pan with cooking spray. Place a piece of parchment paper on the bottom of the pan, letting it hang up and over the sides on the two ends.
In a medium bowl whisk together the cake flour, baking powder, salt, and grapefruit zest. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar on medium speed until light and fluffy, about 3 minutes. Add the eggs one at a time and beat until incorporated, about 1 minute. Add the ricotta, grapefruit juice, and vanilla and beat on medium speed, scraping down the sides of the bowl as needed, about 2 minutes.
With the mixer on low speed, add the flour mixture in three additions. Beat until combined, 1-2 minutes.
Pour the batter into the prepared cake pan. Bake for 55-65 minutes, until a tester inserted into the center of the cake comes out clean. Be sure to insert the tester all the way down to the bottom of the cake, as the top may be done but the bottom might still be gooey.
Let cake cool in the pan on a wire rack for 10 minutes and then remove from pan to cool completely before frosting.