In a large mixing bowl, using a hand held electric mixer, cream butter and brown sugar. Add eggs, grated lemon peel, and vanilla; beat well. Into a separate bowl, sift flour, baking powder, and salt; gradually blend into the creamed mixture. Stir in chopped figs and the chopped nuts, if using. Pour into greased 13X9X2-inch baking pan. Bake fig bars at 350° for 25 minutes. Cool; cut into bars.