In a large bowl, cream the butter, peanut butter, sugar & brown sugar. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, baking powder & salt; gradually add to creamed mixture. Stir in chocolate chips.
Spread into a greased 13" x 9" baking pan. Bake for 45 - 50 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. Cut into bars.