In large bowl, whisk egg yolks and 1/2 cup sugar with an electric mixer until thick and pale yellow, about 3 minutes. In large bowl, combine egg whites, 1/4 cup sugar and salt. Whisk on high speed with electric mixer until foamy; add cream of tartar and vanilla. Continue whisking until the whites are glossy and form stiff peaks, about 5 minutes. Carefully fold 1/3 beaten egg whites into yolk mixture until just combined. Gently fold in remaining egg whites and flour mixture until there are no streaks visible. Spread into prepared pan.