Preheat oven to 350°F. Grease and flour 13x9-inch baking pan. Mix flour, baking powder and salt in medium bowl. Set aside. Microwave butter and milk in medium microwavable bowl on HIGH 2 minutes or until butter is melted. Stir in vanilla. Set aside.
Beat eggs in large bowl with electric mixer on medium speed 5 minutes or until fluffy. Gradually add sugar; beat well. Alternately add dry ingredients and milk mixture, beating on low speed after each addition just until mixed. Pour into prepared pan.
Bake 35 to 40 minutes or until toothpick inserted into center comes out clean. Cool in pan on wire rack. Sprinkle with confectioners' sugar, if desired.