Preheat the oven to 350F. Butter a Bundt pan very well and set it aside.
MAKE THE CAKE:
Cream the butter, sugar, and Kosher salt together in a large bowl.
In a separate container, puree the bananas until smooth. Add eggs and blend well.
Beat the banana-egg mixture into the creamed butter until well-combined. Add vanilla and milk to the bowl and continue blending until fully incorporated.
In a small bowl, combine the flour, baking powder/bicarbonate of soda, and cinnamon. Stir together until uniformly distributed.
Add the dry ingredients to the wet, blending until just combined into a silken smooth batter. Pour the batter into the prepared Bundt pan. Knock the bottom of the pan on the counter a couple of times to settle the batter.
Bake for 25-30 minutes (checking after 20) until the edges are dark golden and the cake passes the toothpick test. Allow cake to cool in the pan for 20 minutes then turn it onto a baking rack to continue cooling.
MAKE THE SAUCE:
Puree the banana until completely smooth.
Add sugar, water, and cinnamon to a medium-sized saucepan with a heavy bottom. Scrape the vanilla bean seeds into the pan and toss the bean pod in as well. DO NOT STIR.
Place the pan over medium-high heat and cook, swirling every few seconds. DO NOT STIR. Continue cooking and swirling until the sugar liquefies and turns a dark golden brown. Remove from heat and use a fork to fish the vanilla bean pod out of the caramel.
Add the banana puree and butter to the warm sauce and quickly stir until fully combined and smooth. If there are lumps of butter you can put the pan over low heat for a few seconds before stirring again. If there are lumps of banana, use a stick blender or whisk to dissolve them.
Pour half of the warm caramel sauce over the cake and wait 30 minutes for it to absorb. Pour the remaining caramel sauce over the cake and serve.