This dessert isn't a pie at all; it consists of two layers of sponge cake with a custard filling. It was Boston's Parker House Hotel that crowned this "pie" with its now familiar chocolate topping. (In the more traditional version, powdered sugar is dusted on top.)
Coat bottom of a 9-inch round cake pan with cooking spray. Dust with 2 teaspoons flour; set aside.
Beat 1/2 cup sugar and margarine in a medium bowl at medium speed of a mixer until well-blended (about 5 minutes). Add vanilla and egg yolk, and beat well. Sift together 1 1/2 cups flour, baking powder, and salt. Add flour mixture to creamed mixture alternately with milk, beginning and ending with flour mixture.
Beat egg whites at high speed of a mixer until foamy using clean dry beaters. Gradually add 3 tablespoons sugar, beating mixture until stiff peaks form. Gently stir one-fourth of egg white mixture into batter. Gently fold in remaining egg white mixture. Pour into prepared pan. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.
Split cake in half horizontally using a serrated knife; place bottom layer, cut side up, on a serving plate. Spread Vanilla Pastry Cream evenly over bottom layer; top with remaining cake layer. Spread Chocolate Glaze evenly over top. Chill 1 hour or until glaze is set.