Step 1Soak poppy seeds in milk 2 hours.
Step 2Cream sugar and butter at medium speed, 2 minutes.
Step 3Sift together flour and baking soda. Add to sugar mixture alternately with milk mixture, beginning and ending with dry ingredients. Mix well. Add vanilla.
Step 4Beat egg whites until stiff but not dry, 2 to 3 minutes. Fold into batter.
Step 5Divide batter between 2 greased and floured 9-inch round cake pans. Bake at 350 degrees until toothpick inserted in center of cakes comes out clean, 25 minutes. Cool in pans on racks, then turn out onto rack.
Step 1Whisk together egg yolks, sugar, cornstarch and milk in saucepan. Bring to boil over medium heat, then simmer, whisking frequently, until filling has consistency of light custard, 2 to 4 minutes. Cool to room temperature (filling will set up more as it cools), then stir in maple extract.
Step 2Spread 1/2 filling across 1 cake, then sprinkle with 1/2 chopped nuts. Place second cake on top, then top with remaining filling and nuts. Cut into slices and serve.