Preheat the oven to 350 degrees F and line a cupcake pan with papers.

Place the flour, sugar, baking soda, and salt in a large mixing bowl, and whisk to combine.  

Place the oil, grapefruit juice, champagne, buttermilk, egg whites, and zest in a large liquid measuring cup, and whisk to combine.  

Pour the liquid ingredients into the dry, and mix on medium speed until smooth.  Scrape the bottom and sides of the bowl with a silicone spatula, to be sure that all the ingredients are well incorporated.

Transfer the batter to the prepared cupcake pan, and bake for 16 to 20 minutes, or until a toothpick inserted into the thickest part of a cupcake comes out clean or with a few moist crumbs.  

Cool completely, then top with frosting and garnish with gold disco dust.

Place the champagne in a small pot, and bring to a boil over medium heat.  Continue to boil until the champagne has reduced to a scant 1/4 cup.  Cool completely and set aside.

Place the egg whites and sugar in a large metal mixing bowl and whisk together.  Set the bowl over a small pot of simmering water, and heat, whisking occasionally, until the mixture becomes translucent and no longer feels gritty when rubbed between the thumb and forefinger.

Remove from heat and whip on high speed until the meringue holds stiff peaks.  Cool completely (There should be no hint of warmth when the hand is place on the side of the bowl.  Place in the refrigerator for 10-20 minutes, if necessary.)  

Add the butter, a tablespoon at a time, while whipping on medium-high.  

When all the butter has been added, stir in the reserved champagne reduction, grapefruit juice, and grapefruit zest.  

Tint with rose pink gel paste color.