16ozunsalted butterroom temperature cut in one inch cubes
For the Grapefruit Curd
Whisk together sugar, cornstarch, and the flour in a heavy-based saucepan.
Add the grapefruit juice and egg yolks and whisk to combine.
Put the saucepan over medium-high heat and cook, whisking constantly until the mixture boils.
Cook while stirring constantly, the mixture will become thicker and clearer.
Whisk in the champagne and allow mixture to boil for 1 minute.
Remove from the heat and stir in the butter.
Transfer to a bowl, cover surface with plastic wrap and chill.
For the Cake
Preheat the oven to 350°F. Butter and flour 4 six inch cake pans
Sift together the flour, baking powder, and salt.
Cream together the butter, sour cream, and grapefruit zest.
Slowly add the sugar, mixing until the mixture is light and fluffy. Scrape down the sides of the bowl. Add the eggs, one at a time, mixing well between additions and scraping down the sides.
Add the sifted dry ingredients alternately with the grapefruit juice to the creamed mixture, beating well on low speed after each addition. Pour equal amounts of the batter into the four prepared cake pans. (I also use cake strips on my pans for more even baking.) Bake until a toothpick inserted in the middle comes out clean, about 22 min. Cool the cake layers in the pans for 10 min. and then loosen the layers by running a knife between the cake and the edge of the pan. Remove the layers from the pans and put them on a rack to continue cooling.
For the Crumbcoat Buttercream
Beat butter until light and fluffy. Add sugar in several batches and beat until well combined. Add in the grapefruit juice and champagne a tablespoon at a time until desired consistency is reached.
For the Italian Buttercream
Beat the egg whites, salt, cream or tartar and 1/3 cup of sugar until soft peaks form.
In a medium saucepan add the remaining sugar and 1/3 cup water then place on low heat.
Stir constantly until sugar melts and becomes clear.
Maintain at medium high heat until temperature reads 240F.
Drizzle the sugar into the mixer immediately.
Run mixer until meringue is body temperature.
Add room temperature butter into running mixer one piece at a time.
Beat until butter is combined and mixture has reached a silky consistency.
I used an 869 tip to pipe the dollops, froze them, and then placed them on top.
If cake seems unstable then add a skewer through the middle and chill in fridge until set. If things are really bad then add a collar of parchment paper and secure with tape or twine to hold the cake together, chill until set.
If you don't want to make Italian buttercream you can use the American variety. I just prefer the smoothness and taste of the Italian variety. I know it's a pain to make two different types of frosting though!
You can make the curd a few days ahead and keep in the fridge in a sealed container until ready to use.
I piped my dollops onto a small piece of parchment paper then froze them and placed on by the finished cake. I do this for swirled dollops so I can have more control and discard the one's I don't like easily. You can TOTALLY pipe them directly onto the cake though.