Cream butter and granulated sugar until light; beat in 1 whole egg and 2 egg yolks.
Into another bowl, sift together the flour, baking powder, and salt. Add to the butter and egg mixture; stir until thoroughly blended. Spread batter in a greased and floured 9x13x2-inch baking pan. Sprinkle batter with chopped pecans, chocolate chips, and marshmallows.
Beat egg whites until stiff; gently stir in brown sugar then spread over bars. Bake at 350° for 30 to 35 minutes. Cool completely; cut into bars.