Melt butter and butterscotch chips over hot (not boiling) water. Remove from heat and stir in brown sugar. Let cool for 5 minute. Blend in eggs and vanilla. Stir in flour, baking powder and salt. Stir in nuts. Spread into greased and ightly floured 9 x 13 inch baking pan. Bake at 350 for about 20 minutes, or until golden brown and inserted toothpick comes out clean. Cool and cut into 2" squares.