In a small bowl, soak poppy seeds in 3/4 cup milk and 1-1/2 teaspoons vanilla for 1 hour. Meanwhile, separate eggs. Let butter and egg whites stand at room temperature for 30 minutes. Set egg yolks aside until needed. Grease the bottoms of two 8x1-1/2- or 9x1-1/2-inch round cake pans. Line pans with waxed paper; grease and lightly flour. Set pans aside. In medium bowl, combine flour, baking powder and salt; set aside.
For filling: In a heavy medium saucepan, combine 1/2 cup sugar and cornstarch. Gradually stir in 1-1/2 cups milk and egg yolks. Cook and stir over medium-high heat until mixture is thickened and bubbly. Cook and stir for 2 minutes more. Stir in 1 teaspoon vanilla. Pour filling into a bowl. Cover the surface with clear plastic wrap. Chill until completely cooled. Do not stir.
For cake: In a large mixing bowl, beat butter with an electric mixer on medium speed for 30 seconds. Gradually add 1-1/2 cups sugar to butter, about 2 tablespoons at a time, beating on medium to high speed until sugar is nearly dissolved. Alternately add flour mixture and poppy seed mixture to butter mixture, beating on low to medium speed after each addition just until combined. Thoroughly wash beaters.
In another large mixing bowl, beat egg whites with an electric mixer until stiff peaks form (tips stand straight). Gently fold beaten egg whites into batter. Evenly divide batter between prepared cake pans. Bake in a 375 degree F oven for 30 to 35 minutes for 8-inch pans or 25 to 30 minutes for 9 -inch pans or until a wood toothpick inserted near the centers of the layers comes out clean.
Cool cakes in pans on wire racks for 10 minutes. Remove cakes from pans. Cool thoroughly on wire racks.
To assemble, carefully split each cake layer horizontally in half. Place one cake layer, cut side up, on a serving plate. Spread with one-third of the filling. Top with second cake layer, rounded (top) side up. Spread with one-third of the filling. Top with 1 more cake layer, cut side up; spread with remaining filling. Top with the remaining cake layer, placing it rounded side up.
For frosting: In a chilled large mixing bowl beat whipping cream and 1/2 cup sugar with chilled beaters of an electric mixer until soft peaks form. Spread top and sides of cake with frosting. Chill up to 2 hours, or until ready to serve.