Preheat oven to 350 degrees F. Prepare a cupcake pan with liners.
In a large bowl, cream together butter and sugar until very light and fluffy. Sift flour, baking powder, and salt together, and then blend into creamed mixture alternately with champagne.
In another large clean bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into batter to lighten it, then fold in remaining egg whites. Fill the cupcake liners about 2/3 full.
Bake at 350 degrees F for 20 minutes, or until a toothpick inserted into the cake comes out clean. With an electric mixer, beat together sugar and butter.
Mix on low until well blended, and then on medium for another two minutes. Add vanilla and champagne, beating on medium for another minute. In a small heavy saucepan, whisk together the eggs and egg yolks.
Whisk in the sugar, then whisk in the citrus juices and zests. Add the butter and whisk constantly over moderately low heat until the mixture thickens, about 8 minutes; do not let it boil.
Immediately strain the curd into a bowl. Press plastic wrap directly on the surface of the curd and cut 6 small steam vents in the plastic. Refrigerate the curd overnight; it can be refrigerated for up to 1 week.