In a saucepan, melt margarine over medium heat. Stir in brown sugar until dissolved. Remove mixture from heat and cool slightly. Add eggs, one at a time, beating well after each addition. Stir in vanilla. In a bowl, stir together flour, baking powder, and salt. Add to mixture in saucepan. Add half of the nuts and stir well. Turn mixture into a greased 9×13-inch pan. Sprinkle with remaining nuts. Bake in a 350 degree oven for 20 to 25 minutes, or until golden brown.