Preheat oven to 350 degrees F (175 degrees C). Grease a 10-inch round cake pan.
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 a 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. Pour into prepared pan.
Bake at 350 degrees F (175 degrees C) for 25 to 30 minutes, or until a toothpick inserted into the cake comes out clean.
