Preheat oven to 350 degrees. Butter and flour a standard bundt pan.

Zest the 2 grapefruits into a large mixing bowl. Add the sugar to the bowl, and using your fingers, rub the zest and sugar together until they’re well-incorporated to infuse the sugar with the grapefruit essential oils.

Halve one of the grapefruits and squeeze its juice into a measuring cup. Add buttermilk to juice in the measuring cup until there is a total of about 2/3 cup of liquid (you may not need the entire 1/2 cup of buttermilk, depending on the size of your grapefruit).Pour the grapefruit juice/buttermilk mixture into the bowl with the zest and sugar and whisk well. Whisk in the eggs and olive oil.

In another bowl, whisk together the flours, baking powder, baking soda and salt. Gradually stir the dry ingredients into the wet mixture. Pour the batter into the prepared bundt pan.

Bake the cake for 50 to 55 minutes, until it is golden brown on top until a toothpick inserted in the center comes out clean. When done, remove the cake to a rack and allow to cool for 10 minutes.

Run a knife around the edges and unmold the cake onto a cake platter.

For the glaze, halve and squeeze the juice from the remaining grapefruit into a small sauce pan. Add the brown sugar and simmer just to dissolve the sugar. Using a toothpick, prick the cake all over, slowly brush the glaze over the entire cake.