Combine milk and poppy seeds. Cover, and let stand overnight in refrigerator.
Cream butter; gradually add sugar, beating well.
Combine flour, baking powder, and salt; add to creamed mixture alternately with milk mixture, beginning and ending with flour mixture. Mix well after each addition. Stir vanilla into batter.
Beat egg whites (at room temperature) until stiff peaks form; fold into creamed mixture.
Pour batter into 4 greased and floured 9-inch round cake pans. Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans 10 minutes; remove layers from pans, and cool completely. Spread filling between layers and on top of cake.