The other day I made this scrumptious banana coffee cake with chocolate chip streusel and do you know what my husband said about it? HE TOLD ME I SHOULD SELL THE STUFF. Sell it. For money. Because it's just that good. This is quite possibly the nicest thing my husband has ever said about my food before.
Preheat oven to 350°F. Butter and an 8x8 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 toothipick inserted in center comes out clean, about 45 minutes. Cool for about 10 minutes and serve.