FOR THE GRAPEFRUIT CURD

1. Bring grapefruit juice to a simmer in a small saucepan over medium-high heat & cook until reduced to 1 cup, about 12–14 minutes. Let cool slightly.
2. Whisk egg yolks, eggs, sugar, and salt in a medium heatproof bowl to combine. Whisk in grapefruit zest, lemon zest, lemon juice, and reduced grapefruit juice. Set bowl over a large saucepan of simmering water. Cook, whisking almost constantly, until curd is thickened, 6–8 minutes. Remove from heat and whisk in butter, adding a few pieces at a time.
3. Transfer to a nonreactive bowl and press a piece of plastic wrap directly onto surface of curd. Chill until set up and very cold, at least 4 hours and up to 12 hours.

FOR THE COCONUT CAKE

1. Heat oven to 325° & grease three 8″ cake pans with butter and line with parchment paper, set aside.
2. Whisk flour, desiccated coconut, baking powder and salt in a bowl, set aside.
3. Place sugar & egg yolks in the bowl of a stand mixer fitted with a paddle attachment & mix until fluffy. Add mayonnaise, coconut milk, buttermilk (or yogurt), coconut & vanilla extracts – mix until well combined. With the motor running, slowly add dry ingredients until batter forms – transfer batter to a bowl & set aside.
4. Clean stand mixer bowl and add egg whites – replace paddle with whisk attachment. Whip whites until soft peaks form and fold into cake batter. Divide batter between prepared pans, smoothing tops with a rubber spatula; drop pans lightly on a counter to expel large air bubbles.
5. Bake until a toothpick inserted in middle comes out clean, about 30-35 minutes. Let cakes cool for 20 minutes in pans, then invert them onto wire racks and let cool.
6. FOR THE COCONUT MERINGUE FROSTING:
7. Make sure the bowl & whisk are very clean and dry. Prepare a double boiler with a pot or heatproof bowl set over a pot of simmering water (make sure it does not touch the water) and place egg whites, sugar, and cream of tartar in the top pot or bowl over simmering water over medium heat. Whisk for 5 minutes, or until sugar has dissolved. Do not let it boil. mixture should be frothy, warm, and not gritty. If you have a candy thermometer, it should read about 140ºf.
8. Remove from heat, transfer to a large heat-resistant bowl (or use the bowl it’s already in) and beat with a stand or hand mixer with whisk attachment until thick, shiny, and stiff peaks form, about 10 minutes or more. Then beat in vanilla & coconut extracts.
9. TO ASSEMBLE CAKE:
10. Lay a cake round on a piece of parchment on a cakestand, evenly spread cake with approximately 1/3-1/2 cup of curd, leaving about 1/2″ around the edge without curd.
11. Spoon about 1 1/2 cups worth of dollops of meringue frosting on top of the curd, use an offset spatula to evenly & carefully spread the meringue over the entire top & sides of the cake.
12. Repeat the above step with the second & third layers of cake.
13. Once you have completed the third layer, use up all of the rest of the frosting to evenly coat the outside of the cake.
14. Use the back of a spoon to decoratively swirl the meringue. You can chill the cake at this point for 30 minutes and then serve. Or if you want the toasted meringue look, use a blow torch to carefully toast the frosting, chill the cake for 30 minutes and then serve.