1. Preheat oven to 375°. Whisk together first 3 ingredients in a small bowl. Whisk together sugar mixture, milk, and 4 egg yolks in a medium-size heavy saucepan until well blended. Cook over medium heat, stirring constantly, 6 to 8 minutes or until thickened. Remove from heat; stir in butter and vanilla.
2. Layer half of vanilla wafers in an 8-inch square baking dish. Top with half of banana slices and half of pudding. Repeat procedure with remaining wafers, banana slices, and pudding.
3. Beat egg whites at high speed with an electric mixer until foamy. Gradually add 3 Tbsp. sugar, beating until sugar dissolves and stiff peaks form. Spread meringue over pudding, sealing to edge of dish.
4. Bake at 375° for 7 to 10 minutes or until golden. Let cool 30 minutes, and serve warm; or chill an additional hour, and serve cold.