1. Preheat oven to 350°.
2. To prepare cake, coat bottom of a 9-inch round cake pan with cooking spray. Dust with 2 teaspoons cake flour; set aside.
3. Weigh or lightly spoon 5 ounces (about 1 1/4 cups) flour into dry measuring cups; level with a knife. Combine flour, baking powder, and 1/4 teaspoon salt in a small bowl, stirring with a whisk. Place 1/2 cup granulated sugar and 1/4 cup butter in a large bowl, and beat with a mixer at medium speed until light and fluffy (about 5 minutes). Add 1 teaspoon vanilla and egg, beating until well blended. Add flour mixture and 3/4 cup milk alternately to sugar mixture, beginning and ending with flour mixture.
4. Place egg whites in a medium bowl; beat with a mixer at high speed until foamy using clean, dry beaters. Gradually add 3 tablespoons granulated sugar, beating until stiff peaks form. Gently fold egg white mixture into batter; 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; run a knife around outside edge. Remove from pan, and cool completely on a wire rack.
5. To prepare filling, combine 1/2 cup granulated sugar, cornstarch, and 1/8 teaspoon salt in a medium saucepan. Gradually add 1 cup plus 2 tablespoons milk and egg substitute to pan, stirring with a whisk until well blended. Bring to a boil over medium heat, stirring constantly with a whisk until thick. Remove from heat; stir in 1 tablespoon butter and 1/2 teaspoon vanilla. Place pan in a large ice-filled bowl until custard cools to room temperature (about 15 minutes), stirring occasionally.
6. To prepare glaze, place chocolate and 2 tablespoons milk in a microwave-safe bowl. Microwave at HIGH 20 seconds or until chocolate melts. Add powdered sugar and liqueur, stirring with a whisk until smooth.
7. Split cake in half horizontally using a serrated knife; place bottom layer, cut side up, on a serving plate. Spread cooled filling evenly over bottom layer; top with remaining cake layer, cut side down. Spread glaze evenly over top cake layer.