Combine 1/2 cup sugar, flour and salt in top of a double boiler; mix well. Stir in 1 egg, 3 egg yolks and milk; mix well. Place over boiling water; cook, stirring constantly, until smooth and thickened. Remove from heat; stir in vanilla. Spread about 1/4 cup pudding in a 1 3/4 quart casserole; arrange 1 dozen vanilla wafers over top of pudding. Top with 1/3 banana slices and 1/3 of remaining pudding. Repeat layers twice. Beat 3 egg whites (at room temperature) at high speed 1 minute. Gradually add 1/4 cup sugar a tablespoon at a time, beating until stiff peaks form and sugar dissolves ( 2 - 4 minutes). Spread meringue over pudding, sealing to edge of casserole. Bake at 350°F for 12 to 15 minutes or until golden brown.