Preheat your oven to 350°F and grease your springform pan, lining the bottom with parchment paper.
Zest your grapefruit until you have about 2 tablespoons, avoiding the white part of the peel.
Juice enough grapefruit to yield 1 cup of liquid, about one and a half grapefruits.
In a medium bowl, sift together flour, baking powder, and sea salt. Stir in grapefruit zest.
In a large bowl, whisk eggs until mixed, then add sugar and vanilla extract, whisking until fluffy.
Alternate adding dry ingredients and grapefruit mixtures into the egg mixture while whisking continuously.
Pour the batter into the prepared springform pan.
Bake for about 50–60 minutes, checking for doneness with a toothpick.
Allow the cake to cool slightly before inverting onto a serving platter.
Serve with a dusting of powdered sugar and fresh grapefruit sections.
