Melt the butter in a large saucepan over medium heat; stir in the brown sugar, mixing well. Remove from heat and set aside to cool for 10 minutes.
Beat in the eggs, one at a time; add the flour, salt and baking powder. Stir in chips and nuts. Spread evenly into a greased 10 x 15-inch jellyroll pan; press dough down evenly with moistened fingers. Bake until a tester comes out clean and sides begin to pull away from the pan, about 25-30 minutes. Transfer pan to a wire rack to cool. cut into bars.