Preheat the oven to 350 degrees F. Grease a 9x13 baking dish.
Mix the flour, cinnamon, nutmeg, baking soda, baking powder and salt together in a large bowl. In another bowl, mash the bananas with your fork or a blender. Add in the sugar and vanilla and mix to combine. Pour in the milk and stir just to combine.
Working in batches, mix the flour mixture into the banana batter until just combined. Stir in the dark chocolate chips and pour into the greased baking dish.
In another bowl, mix the flour, salt, cinnamon, brown sugar and diced butter together. Use your fingers to make quick work of breaking down the butter into smaller lumps. Add the oats and walnuts in and give another quick stir with your hand, working it all together, but still allowing it to remain a bit clumped. Pat the topping gently all over the top of the batter.
Slide the cake pan into the oven and bake for 40 to 45 minutes.
Remove from oven and allow to cool on a wire rack for 20 minutes before slicing or serving.