In a small bowl, cream the butter, 1/3 cup brown sugar and sugar until light and fluffy. Beat in vanilla. Combine the flour, baking soda and salt; add to the creamed mixture alternately with milk, beating well after each addition. Press into a 13x9-in. baking pan coated with cooking spray. Sprinkle with chocolate chips, coconut and walnuts.
In a large bowl, beat egg whites and cream of tartar until soft peaks form. Gradually beat in remaining brown sugar, 1 tablespoon at a time. Beat until stiff peaks form. Spread over the top. Bake at 350° for 30-35 minutes or until a toothpick inserted in the center comes out clean (do not overbake).
Cool on a wire rack. Cut into bars. Store in the refrigerator.