Preheat oven to 350 degrees.

Butter and flour 8-inch square metal baking pan. Stir chocolate chips, brown sugar, walnuts and cinnamon in a small bowl, set aside. Sift flour, baking soda, baking powder and salt into medium bowl. Beat sugar, butter and egg in a large bowl until fluffy. Beat in mashed bananas and buttermilk. Add dry ingredients and blend well.

Spread half of batter (about 2 cups) in pan. Sprinkle with half the streusel. Repeat with remaining batter and streusel. Bake about 45 minutes or until tester comes out clean.