Preheat oven to 325°
Using a silicone pastry brush, dot and spread one tablespoon of the softened butter to every fold and crease in the Bundt pan. Be sure to butter the entire pan. Flour liberally tapping out any excess. Set aside.
In a large bowl sift together the flour, baking powder, and salt. Set aside.
Using a vegetable peeler, peel the zest of the grapefruit. Put the zest and 2/3 of the sugar in a spice grinder or mini chef prep and pulse until the zest is combined with the sugar. Reserve 1 heaping tablespoon of the grapefruit sugar.
Place the softened butter, cream cheese and both the 1 cup of granulated sugar and the remaining grapefruit sugar in the bowl of a stand mixer; beat together until the mixture is light and fluffy.
Add in eggs one at a time, mixing after each addition. Be sure to scrape the bowl using a spatula to make sure all ingredients are sufficiently incorporated. Beat in oil and vanilla.
Add the flour mixture and the buttermilk alternately to the batter, starting and ending with the flour. Again be sure to scrape the bowl using a spatula to make sure all ingredients are sufficiently incorporated.
Spoon batter into prepared pan and bake at 325° for one hour or until a toothpick inserted in the center comes out clean. Let the cake cool for 10 minutes then invert on to your desired cake plate or stand.