Begin by preheating the oven to 350 degrees Fahrenheit. Lightly grease (w/ butter or spray) and line the bottom with parchment paper of a 9-inch round cake pan.
In a bowl, whisk together the flour, baking powder, grapefruit zest, and sea salt until combined. Set aside.
In a large bowl, add together the sugar and unsalted butter, mixing by using a hand-mixer on medium-high speed, until fluffy and airy, about 2-3 minutes.
Add in the eggs (one at a time), vanilla, grapefruit juice, olive oil, and the 'buttermilk', followed by the dry ingredients, mixing everything together until well combined, smooth, and lump-free, about 2-3 minutes.
Pour batter evenly into prepared pan, spreading everything around for a perfect evenness.
Bake for 55-60 minutes, or until the center of the cake comes out clean when tested with a toothpick or butter knife.
Once done, remove the cake from the oven and let cool for 30 minutes before carefully lifting it from the pan and unto a cooling rack to cool completely, about 1 ½ to 2 hours.