Preheat the oven to 350°F and line two muffin tins with liners. Meanwhile, beat the butter for two minutes on high until light and fluffy. Add the sugar and beat two minutes more.
In a separate mixing bowl, combine cake flour, salt, and baking powder. Set aside.
Beat the egg whites into the butter mixture, one egg at a time. Add the vanilla. Alternate adding the flour mixture and champagne until everything is just combined, but don't overbeat.
Scoop the batter just over 2/3 full and bake for 16-18 minutes. If you do mini cupcakes, they'll bake 10-11 minutes.
For the raspberry champagne frosting:
While the cupcakes are baking, reduce the champagne for the frosting. In a small saucepan, bring the 3/4 cup champagne to a boil. Once it bubbles, turn the heat to low and simmer until it's only about 1/4-1/3 cup. Pour into a glass or other room temperature dish and let cool completely.
Puree the raspberries: use a spoon to smash the raspberries through a sieve. This lets the liquid get through without seeds. It makes about 3-4 Tbsps.
Beat the butter on high for 2-3 minutes. Add 1 cup of confectioner's sugar and beat until fully combined.
Add the vanilla, 2 Tbsp of the raspberry puree and 2 Tbsp of the champagne. Beat until combined, then 3 more cups confectioner's sugar.
Taste and add more raspberry puree, champagne, and sugar until the flavor and consistency are what you want. I used all the puree (3.5 Tbsp) and champagne (4 Tbsp), and just under 5 cups of sugar.
Make sure the cupcakes are fully cool before frosting. Top with raspberries for an extra touch!