Heat oven to 350 Spray a 10" bundt pan w/ cooking spray. In small bowl, combine 1st 4 ingredients, set aside. In lg bowl whisk together buttermilk, oil, sugar, eggs and vanilla.
Beat in remaining dry ingredients until combined. Stir in bananas. Spoon 1/3 cup batter into pan. Sprinkle w/ 1/2 the walnut/chocolate chips mix.
Spoon another 1/3 cup batter over walnut mix, sprinkle w/ remaining walnut mix. Bake about 50 mins, or until a toothpick inserted in center comes out clean. While cake cools, Melt remaining chocolate chips and butter in microwave, stirring until melted. Let cool slightly, then drizzle over cake.