Combine brown sugar and butter in a saucepan over medium heat; cook until butter is melted. Remove from heat. Cool slightly; add eggs, 1 at a time, beating until blended after each addition. Combine flour, baking powder, and salt in a bowl; stir into brown sugar mixture. Stir in peanut brittle and vanilla. Press dough into a greased and floured 13" x 9" baking dish.
Bake at 350° for 25 minutes or until golden. Cool in pan on a wire rack. Cut into bars.