Preheat the oven to 350 degrees Fahrenheit/180 degrees Celsius.
Prepare a 9- or 10-inch cake pan by spraying or brushing on oil on the bottom and sides. I used a springform pan, which make unmolding a little easier, but you can use a regular cake pan. Line the bottom of the pan with parchment paper.
Mash or puree the four overripe bananas until quite smooth. You can do this in a food processor or blender, which just makes the process easier, or with a fork. Set aside.
In a large bowl whisk together the sugar and coconut oil for about a minute.
Add the milk, bananas, apple cider vinegar and vanilla and whisk until mixed.
Sift the three cups of flour into the bowl, followed by the baking powder and baking soda. Add salt and cinnamon, if using.
Use a spatula to mix the dry ingredients into the wet ingredients until just incorporated. Don't overmix but make sure there are no dry streaks of flour in the bowl.
Stir in the toasted, chopped walnuts.
Sprinkle the remaining two tablespoons of sugar on the bottom of the cake pan.
Slice the one remaining banana and arrange the slices over the sugar.
Pour the batter into the cake pan (it will be quite thick) and smooth the top with a spatula or spoon.
Bake 35 minutes or until a toothpick in the center comes out clean.
Cool the cake on a rack for 20 minutes. Run a knife along the edges to loosen from the pan, then place a plate over the pan and flip it quickly so the cake unmolds. If you used a springform pan, release it, and if you used a cake pan lift it off. Gently peel off the parchment paper. Continue cooling the cake on a rack.