Preheat oven to 300° F. Sift 1/4 c. of sugar, Rumford Baking Powder, and flour together three times. Beat egg whites, cream of tartar, salt and vanilla extract until frothy. Add remaining 3/4 c. of sugar, then continue beating until stiff peaks form, but not dry. Fold in the flour mixture, about 3 or 4 Tbsp. at a time until all has been added. Pour into ungreased angel food cake pan and bake for 1 hour and 15 minutes. Invert pan on wire cake rack until cool. Leave plain or frost with your favorite frosting.