For The Cake, First Wash And Pat The Grapefruit Dry.
Zest The Grapefruit To Yield 1 Tablespoon Finely Grated Grapefruit Zest, And Set This Aside For The Frosting.
With A Sharp Knife, Cut The Skin And Pith Off The Grapefruit.
Hold The Fruit Over A Bowl To Catch The Grapefruit Juice.
You Will Use This Juice In The Cake, And You Need A Scant ½ Cup Juice.
If You Don’t Collect Enough Juice, Make Up The Difference In Water.
Set The Juice Aside For The Cake.
Meanwhile, Carefully Cut The Peeled Grapefruit Into Sections, Removing The Membrane And Keeping The Slices Whole.
Set The Slices Aside In A Sieve To Drain, And Then Pat Them Dry With Paper Towels.
You Will Use These Between Layers And On Top Of The Finished Cake.
Place A Rack In The Center Of The Oven, And Preheat The Oven To 350°F.
Set Aside An Ungreased 9″ Springform Pan.
Sift Together The Flour, Sugar, Baking Powder, And Salt Into A Large Mixing Bowl.
Add The Oil, Egg Yolks, And Lemon Zest.
Add The Reserved Scant ½ Cup Grapefruit Juice.
Blend With An Electric Mixer On Medium Speed Until Smooth, About 1 Minute.
In A Large Bowl, Place The Egg Whites And Cream Of Tartar.
Beat With Clean Beaters On High Speed Until The Whites Come To Stiff Peaks, But Are Not Dry, 3 To 4 Minutes.
Fold The Egg Whites Into The Batter Until Just Blended.
Turn The Batter Into The Springform Pan, And Place The Pan In The Oven.
Bake Until The Cake Is Golden Brown And The Top Springs Back When Lightly Pressed In The Center, 28 To 32 Minutes.
Remove The Cake From The Oven, And Let It Cool Completely In The Pan, About 1 Hour.
Meanwhile, Prepare The Frosting.
Place The Cream Cheese And Butter In A Large Mixing Bowl, And Blend With An Electric Mixer On Medium-Low Speed Until Creamy, About 30 Seconds.
Add The Confectioners’ Sugar, The Reserved Grapefruit Zest, And Vanilla, And Blend Until Smooth, 30 To 45 Seconds More.
Increase The Speed To Medium And Beat Until Fluffy, About 1 Minute.
Set Aside.
When The Cake Has Cooled, Run A Knife Around The Edges, And Remove The Collar Of The Springform Pan.
Carefully Slice The Cake Away From The Bottom Of The Pan, And Place The Cake On A Wire Rack.
With A Serrated Knife, Carefully Slice The Cake Crosswise To Make 2 Layers.
Place The Bottom Layer On A Cake Plate, And Spread It With About ½ Cup Of Frosting, Spreading To The Edges.
Place About 12 Or 13 Sections Of Grapefruit On Top Of The Frosting, Placing The Sections On In 2 Concentric Circles.
Press Down On The Sections So They Stick To The Frosting.
Flip The Top Cake Layer Over To Expose The Cut Side, And Spread This With About â…“ Cup Of Frosting.
Flip This Layer Frosting Side Down Onto The Grapefruit.
With The Remaining Frosting, Frost The Top And Sides Of The Cake Using Clean Strokes.
Arrange The Remaining 7 To 8 Slices Of Grapefruit In A Spiral, Or Arrange As You Wish, On Top Of The Cake.
If Desired, Sprinkle Grapefruit Zest On Top.
Slice And Serve.
