Preheat oven to 350F degrees. Generously grease a 9-inch cake pan with butter, place parchment in bottom of pan and grease the parchment.
Melt 4 tablespoons butter. Mix well with the 2/3 cup brown sugar. Pour this mixture into the bottom of the cake pan and spread to the edges. If including walnuts, sprinkle them over the brown sugar/butter mixture.
Slice the just-ripe bananas in about 1/4-inch slices and place them side-by-side evenly over the brown sugar and walnuts. Usually 2 bananas is enough for this depending on their size.
In a medium bowl, combine the flour, baking powder, and salt. Whisk together and set aside.
In the bowl of a stand mixer place the 2 ripe bananas and grated zucchini, mix until creamy (or mash together by hand in a medium bowl).
Add the egg while the mixer is on low speed, then add the sugars, vanilla, and melted butter. Continue to mix on low until the sugar fully incorporated and dissolved.
Add the buttermilk and turn up the speed to mix on medium for another 30 seconds.
Turn the mixer back to low and add the flour mixture. Mix until fully incorporated, about 30 seconds.
Turn off the mixer and add desired amount of chocolate chips (I highly suggest using the mini chips) and fold them into the batter until evenly distributed.
Pour the batter over the bananas in the cake pan. Spread to edges.
Bake for 42-45 minutes or until a toothpick or cake tester comes out barely clean.
Place cake pan on a rack and cool for just 5 minutes. Then invert onto serving platter and do not remove the pan for about a minute, allowing the glaze to completely absorb onto the cake.
Remove pan and serve warm or at room temperature. Freshly whipped cream and extra banana slice can be used to top the cake but it's fabulous (fabulous!) with no toppings at all.
Wrap any leftovers in plastic wrap, can be left at room temperature overnight.
Recommend eating within 24 hours of baking.