A very tasty cake, not overwhelmingly "grapefruit-y". Like KrabKokonas, I would recommend doubling the peel for more citrus flavor and increasing the % of grapefruit juice. Didn't have any buttermilk, so combined whole milk with the juice to sour it. We found that this cake was better eaten the first day or two.
I followed the directions exactly as written and it made one GREAT cake! I will definitely be making this again very soon. A couple of minor comments: There was very little grapefruit taste to the cake itself. The poppy seed taste, however, was excellent. If you want a more citrusy cake, I would recommend doubling the peel to be put into both the batter and the glaze. I might also substitute a quarter cup of the buttermilk for some more grapefruit juice. Thanks for a real keeper, RÃ©eLani! UPDATE: I made this again (it's really that good:) and this time made a citrus buttercream frosting for the top and decorated it with starfruit. I didn't think this cake could get any better, but the frosting really added a nice zing!