Stir together butter and brown sugar. Add egg, stirring until blended.
Combine flour, baking powder, and salt; add to egg mixture, stirring well. Stir in vanilla and pecans. Pour mixture into a greased and floured 8-inch pan.
Bake at 350° for 30 to 35 minutes or until set. Cool on a wire rack. Cut into squares, and sprinkle with powdered sugar.