Preheat oven to 350°. In a large bowl, beat eggs and sugar until thick and lemon-colored. Beat in milk and vanilla. In another bowl, combine flour, baking powder and salt; gradually add to egg mixture and mix well., Spread into a greased 15x10x1-in. baking pan. Bake 20-25 minutes or until lightly browned. Cool 15 minutes on a wire rack. Spread peanut butter over top; cool completely., In a double boiler or metal bowl over simmering water, melt chocolate and butter; stir until smooth. Gently spread over peanut butter. Refrigerate 30 minutes or until firm.