Soak poppy seed in milk for 2 hours.
Sift the flour, baking powder, and salt together; set aside.
Cream butter with extract.
Gradually add sugar, creaming until fluffy.
Beating only until smooth after each addition, alternately add dry ingredients in fourths and poppy seed-milk mixture in thirds.
Gently fold egg whites into batter until blended.
Turn batter into a greased (bottom only) 13x9x2-inch baking pan.
Bake at 350CF 25 to 30 minutes, or until cake tests done.
Cool and remove from pan as directed for butter-type cakes.
Using a long thin sharp knife, cut cake in half lengthwise and then in half cross wise, making four 9x61/2 inch cake layers.
Spread about 6 tablespoons Vanilla Custard Filling on one layer and sprinkle with about 1/4 cup of walnuts.
Repeat layering, ending with filling and nuts.