Make crepe batter: Combine water, milk, eggs, flour, vanilla, sugar and salt in a blender. Blend until all of the ingredients are well incorporated and the mixture is no longer lumpy. Scrape down the sides and blend again. Add the butter to the mixture and blend for just a few more seconds. Let the mixture rest in the fridge for an hour. In the meantime, make filling and whipped cream.
Make filling: In a sauté pan over medium-low heat, melt butter in the pan. Once the butter has melted, add the sugar, cinnamon and vanilla to the pan. Stir until dissolved and add bananas. Whenever the slices begin to brown and soften, carefully add the rum (if using.) Light the pan with a long match or a torch. Allow the flames to subside. Let cool and mash with the back of a fork or a potato masher until thick and almost a puréed consistency.
Make whipped cream: Whisk all of the ingredients in a stand mixer until soft peaks form. The whipped cream should be able to hold a peak if inverted.
Cook crepes: Place a nonstick skillet on medium to medium-low heat. Brush butter onto the pan. With the batter in one hand and the hot skillet in the other, slowly pour the batter into the pan and twirl the skillet in the other so that there is enough batter to purely coat the bottom of the pan. Let the crêpe cook just until brown at the edges, flip with a spatula and brown on the other side. Flip onto a plate and continue with the remaining batter.
Assemble cake: On a cake stand, place one crepe on the bottom of the stand. Spread a thin layer of the bananas foster puree onto the crepe. Repeat with remaining layers until you add the last crepe (or run out of filling! Whichever comes first.) Finish the last crepe with a topping of the rum whipped cream and adorn with caramelized bananas, if desired**.
**Note: For the caramelized bananas, sprinkle sugar onto 1''€ slices of banana. Using a torch, carefully caramelize the top of the banana like you would a creme brulee.