Dissolve yeast in warm milk, stirring well. Let stand 5 minutes or until bubbly.
Cream butter; gradually add sugar, beating well. Add egg yolks, beating until smooth.
Combine flour and salt; add to creamed mixture alternately with yeast mixture, beginning and ending with flour mixture. Beat egg whites (at room temperature) until stiff peaks form; fold into batter.
Spoon batter into a well-greased 9-cup mold. Let rise, uncovered, in a warm place (85°), free from drafts, 1 hour or until doubled in bulk.
Bake at 375° for 30 minutes or until golden brown. Cool in mold 10 minutes; invert onto serving plate. Slice and serve.