Preheat the oven to 350 F. Grease a 12-cup muffin pan unless using non-stick. Cream the butter and sugar in a large bowl for several minutes until light pale and fluffy. Add the egg yolks and beat until combined. Add the grapefruit juice, zest, and vanilla extract and beat until combined.In a separate bowl, combine the flour, baking powder, baking soda and salt. A bit at a time, carefully fold the flour mixture into the batter using a rubber spatula. Add the buttermilk between flour additions. Some lumps are fine. Be careful not to over-stir.
Beat the egg whites until soft peaks form. Using a rubber spatula, carefully fold the egg whites into the batter, again being careful not to over-stir. Spoon the batter into the muffin tins about 3/4 full.
To make the grapefruit sugar, use your fingers to combine the grapefruit zest with the sugar in a small bowl. Sprinkle the grapefruit sugar on top of the muffins.Bake for 20-25 minutes or until a toothpick inserted into the center of a muffin comes out clean. Let the muffins sit for 5 minutes, then transfer them to a wire rack.