Place egg whites in a small bowl; let stand at room temperature for 30 minutes. Coat three 9" round baking pans with cooking spray; line with waxed paper & coat the paper. Sprinkle with flour; set aside.
In a large bowl, beat butter & sugar blend until crumbly, about 2 minutes. Beat in applesauce & 1 tsp vanilla. Combine 2 1/2 C flour, baking powder, baking soda & salt; add to creamed mixture alternately with buttermilk. Fold in poppy seeds.
Beat egg whites on medium speed until soft peaks form; fold into batter. Spread into prepared pans. Bake at 350 degrees F. for 12 - 18 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely.
For filling, in a small saucepan, combine sugar & remaining flour. Stir in sour cream until smooth. Bring to a boil. Stir a small amount of hot mixture into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook & stir 2 minutes longer. Remove from the heat. Stir in remaining vanilla. Transfer to a bowl; refrigerate until chilled.
Place one cake layer on a serving platter; top with a 1/3 of the filling. Repeat layers twice.
Combine the confectioners' sugar, cocoa & enough coffee to achieve a drizzling consistency. Drizzle over cake. Sprinkle with nuts. Refrigerate until serving.