1 1/4 Cup(s) semisweet chocolate chips (about 8 oz.)
2/3 Cup(s) (packed) golden brown sugar
1/2 Cup(s) chopped walnuts
1 Tablespoon(s) ground cinnamon
1 1/2 Cup(s) all purpose flour
3/4 Teaspoon(s) baking soda
3/4 Teaspoon(s) baking powder
1/4 Teaspoon(s) salt
3/4 Cup(s) sugar
1/2 Cup(s) unsalted butter, room temperature
1 Each(s) large egg
1 3/10 Cup(s) mashed very ripe bananas (about 3 large)
3 Slice(s) buttermilk
Preheat oven to 350F. Butter and flour 8x8x2-inch metal pan. Stir chocolate chips, brown sugar, walnuts, and cinnamon in small bowl until well blended; set streusel aside. Sift all purpose flour, baking soda, baking powder, and salt into medium bowl. Using electric mixer, beat sugar, room temperature butter, and egg in large bowl until fluffy. Beat in mashed bananas and buttermilk. Add dry ingredients and blend well.
Spread half of batter (about 2 cups) in prepared baking pan. Sprinkle with half of streusel. Repeat with remaining batter and streusel. Bake coffee cake until tester inserted into center comes out clean, about 45 minutes. Cool coffee cake in pan on rack.