Preheat oven to 350º

Zest one of the grapefruits and set the zest aside - you want a good heaping tablespoonful.

Peel and slice the grapefruits as described in the post.

Place the first amount of butter [2 tablespoons] in a 9 inch round cake pan, and place in the oven until the butter melts.

Once the butter is melted, swirl to get an even layer in the pan, and the sprinkle the 1/3 cup of brown sugar over the butter.

Lay the slices of grapefruit over the butter and sugar - the slices will overlap, which is fine.

Sprinkle the 1/8 teaspoon of kosher salt evenly over the grapefruit slices. Set aside while you make the batter.

In a small bowl, whisk together the flour, baking powder, salt and cardamom, set aside

Cream together the 6 tablespoons of butter and the 3/4 cup brown sugar.

Beat in the eggs, one at a time.

Beat in the vanilla, ginger and grapefruit zest.

Beat in the flour, in 3 additions, alternating with the buttermilk: flour, buttermilk, flour, buttermilk, flour. You don't want to overmix, so kind of overlap the additions a little - they don't have to be completely incorporated each time, until the last addition, which I usually finish up with a rubber spatula.

Spread the batter evenly over the grapefruit slices, taking care not to disturb them.

Bake for 50 to 55 mins, turning halfway through. The cake should test done in the center with no crumbs sticking to a toothpick, and edges pulled away from the pan slightly.

Cool for about 5 minutes on a wire rack, and then loosen the edges with a butter knife or offset spatula and invert carefully on a serving plate. Cut into wedges to serve warm with lightly sweetened whipped cream or vanilla ice cream. Refrigerate leftovers.