Here's a variation that's served me well, my edit of a recipe from the January 2003 edition of Bon Appétit:
BANANA COFFEE CAKE WITH CHOCOLATE CHIP STREUSEL
1+1/4 cups semisweet chocolate chips (about 8 ounces)
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
1/2 cup (1 stick) unsalted butter, room temperature
3/4 cup sugar
1 large egg
3 tablespoons buttermilk
1+1/3 cups mashed very ripe bananas (about 3 large bananas, very ripe with black spots on the skin)
1. Preheat oven to 350°F.
2. Butter and flour 8x8x2-inch metal baking pan.
3. Stir chocolate chips, brown sugar, walnuts, and cinnamon in small bowl until well blended; set streusel aside.
4. Sift all purpose flour, baking soda, baking powder, and salt into medium bowl.
5. Using electric mixer, beat sugar, room temperature butter, and egg in large bowl until fluffy.
6. Beat in mashed bananas and buttermilk.
7. Add dry ingredients and blend until just incorporated.
8. Spread half of batter (about 2 cups) in prepared baking pan.
9. Sprinkle with half of streusel.
10. Repeat with remaining batter and streusel.
11. Bake coffee cake until tester inserted into center comes out clean, about 45 minutes.
12. Cool coffee cake in pan on rack.
Makes 12 servings.