Source: Bon Appetit - January 2003
Use bananas with some black spots on the skin, a sign that they are really ripe.
1 1/4 cups semisweet chocolate chips (about 8 ounce)
2/3 cup packed golden brown sugar
1/2 cup chopped walnuts
1 tablespoon ground cinnamon
1 1/2 cups all-purpose flour
3/4 teaspoon baking soda
3/4 teaspoon baking powder
1/4 teaspoon salt
3/4 cup granulated sugar
1/2 cup (1 stick) unsalted butter, room temperature
1 large egg
1 1/3 cups mashed very ripe bananas (about 3 large)
3 tablespoons buttermilk
Preheat oven to 350 degrees F. Butter and flour an 8-inch square baking pan.
Stir chocolate chips, brown sugar, walnuts and cinnamon in small bowl until well blended; set streusel aside.
Sift 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.