Heat oven to 350. Grease bottom Only of 8-inch square pan.
Melt in 2-quart saucepan, over low heat, stirring constantly; 1/2 stick butter (1/4 cup) 1 pkg. (6 oz.) semi-sweet chocolate chips Beat in thoroughly with spoon; 3/4 cup all-purpose flour 1/2 teaspoon vanilla 1/2 cup sugar 1/4 teaspoon baking powder 1/3 cup chopped walnuts 1/4 teaspoon salt 1/3 cup mashed ripe banana 1 egg (about 1 small) Spread batter evenly in pan. Bake about 30 minutes or until center is set. Cut into 2-inch squares.