Preheat oven to 350°F. Butter and an 8×8 inch baking dish.
Stir chocolate chips, brown sugar, walnuts and cinnamon in small bowl until well blended, \set streusel aside.
Sift together (or just STIR TOGETHER) all purpose flour, baking soda, baking powder and salt into medium bowl. Beat sugar, butter and egg in a large bowl until fluffy. Mix in mashed bananas and buttermilk or yogurt. Add flour mixture and blend well.
Spread half of batter in prepared baking pan. Sprinkle with half of streusel. Repeat with remaining batter and streusel. Bake coffee cake until toothpick inserted in center comes out clean about 45 minutes. Cool for about 10 minutes and serve.