Step 1: Preheat oven to 350 degrees F. Prepare a cupcake pan with liners.
Step 2: 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.
Step 3: 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.
Step 4: 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.
Step 5: 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.
Step 6: 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.
Step 7: 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.