Dissolve brown sugar in melted butter; allow to cool slightly. Add eggs, flour and baking powder. Add nuts and vanilla; mix well. Pour into well greased 9 x 14-inch pan. Bake at 350 degrees for 20 to 30 minutes. Allow to cool slightly before cutting. These squares can be frozen in 1 long piece or 2 large squares and cut into desired sizes when ready to serve. Makes about 48 squares.