1. Preheat oven to 350°F.
2. Spray or grease a 9 inch round cake pan and sprinkle lightly with powdered sugar (just to coat, no excess). In a medium bowl, whisk together dry ingredients: flour, baking powder, salt, ginger, and nutmeg. Set aside.
3. In a large bowl (or the bowl of a stand mixer), add the sugar. Zest the grapefruit over the sugar and working with hands or a fork, rub the mixture together until fragrant and evenly distributed about 30 seconds.
4. Add in Eggland's Best eggs, sour cream, olive oil, grapefruit juice, and vanilla. Beat on medium-low speed until thin and smooth (ingredients should be fully incorporated and a few bubbles will appear).
5. Using a spoon or rubber spatula, fold in the dry ingredients until just combined (no visible streaks of flour remain; batter may contain a few lumps-- don't overmix).
6. Pour batter into prepared pan and bake for about 35 minutes, or until crust and top are golden brown and toothpick inserted comes out clean. *depending on your oven and baking pan, bake time may be between 30 to 40 minutes.
7. Let cake cool in pan for 15 minutes, then remove to a cooling rack. Let cool completely before dusting with powdered sugar, if desired.

Recipe Notes: Serve cake slices with powdered sugar and fresh fruit, if desired. If topping with fresh grapefruit segments, only do so just before serving (to prevent cake top from sweating). Cake will keep covered at room temperature for up to 4 days.