Place egg whites in a large bowl; let stand at room temperature for 30 minutes.
In a large bowl, combine the cake mix, milk, buttermilk, oil, applesauce, lemon juice, egg yolks, lemon peel and poppy seeds; beat on low speed for 30 seconds. Beat on medium for 2 minutes. In another bowl, beat egg whites until stiff peaks form; fold into batter.
Transfer to two greased and floured 9-in. round baking pans. Bake at 350° for 22-28 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.
In a large bowl, beat cream until it begins to thicken. Add sugar and vanilla; beat until stiff peaks form.
Cut each cake horizontally into two layers. Place bottom layer on a serving plate; top with 1/3 cup lemon curd. Repeat layers twice. Top with remaining cake layer. Spread remaining lemon curd over top of cake within 1 in. of edges. Frost sides and top edge of cake with 3 cups whipped cream mixture. Pipe remaining mixture around edges. Yield: 12 servings.