Sift together 3 times, cake flour, baking powder and ¼c. sugar.
Combine eggs, salt, cream of tartar and vanilla in large mixer bowl. Beat at high speed on electric mixer, until mixture stands in peaks. Add ½ c. sugar, 2 tblsp. at a time. Continue beating at high speed until mixture again stands in peaks.
Carefully fold dry ingredients into egg mixture.
Grease a 15½ X 10½ X 1" jelly roll pan and line with waxed paper so that paper extends above sides. Pour in batter. Bake in moderate oven (375°) 13 minutes. Cool on rack 1 minute.
Loosen sides and turn out onto towel sprinkled with confectioners sugar. Trim edges with sharp knife. Peel off waxed paper. Fold towel over long side of cake and roll cake in towel to “set” roll while cake is warm.
Cool, unroll and spread with Chocolate Filling to within 1" of edges. Roll up cake and chill 2 to 3 hours. Slice into 10 servings.