Beat 4 eggs in the top of a double boiler, just until blended. Add 2 cups brown sugar and 1 tablespoon butter. Place over rapidly boiling water, stirring constantly until HOT, about 5 minutes. Remove from heat. Sift together 1 1/2 cups flour and 1 l/2 teaspoons baking powder; add to mixture. Mix until blended. Stir in 1 l/2 cups chopped nuts and 1 teaspoon vanilla. Put in 9"x13" pan and bake at 350° for 25 to 30 minutes. Serve with whip cream, ice cream or plain. Good for lunch boxes.