In a large bowl, cream butter and sugar until light and fluffy. Beat in egg yolks. Beat in vanilla. Combine the flour, poppy seeds, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk.
In a small bowl, beat egg whites until stiff peaks form; fold into batter. Pour half of the batter into a greased 13-in. x 9-in. baking pan.
Combine filling ingredients; sprinkle over batter. Carefully top with remaining batter. Bake at 350° for 35-40 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. Yield: 12-15 servings.