Preheat oven to 350℉ (180℃)
Prepare an 8" round cake pan by lining the bottom with parchment paper or greasing the pan.
Smash one banana and set aside.
1 Ripe Banana, smashed
Whisk flour, baking powder, salt and cinnamon in a small bowl. Set aside for later.
1 ½ Cups All Purpose Flour, 1 ½ teaspoon Baking Powder, 1 teaspoon Salt, 1 teaspoon Ground Cinnamon
Place butter and granulated sugar the bowl of a stand mixer. Using the paddle attachment, begin mixing the butter and sugar until light and fluffy (2 - 4 minutes), scraping sides and bottom of bowl occasionally. Note: a hand mixer may also be used.
¾ Cup Butter, ⅔ Cup Granulated Sugar
Add room temperature eggs, one at a time and mix until well blended. Scrape down sides and bottom of bowl between each addition. Mix on medium-low speed until eggs are well incorporated.
3 Large Eggs
Add rum and mix for approximately 30 seconds, Note - rum is optional, you may replace with 1 teaspoon of vanilla extract.
2 Tablespoons Dark Rum
Add smashed banana and mix for approximately 60 seconds.
2 Tablespoons Bananas
Add flour mixture and mix on low speed until just combined. Take care to not overmix which may result in a tough cake texture.
Pour half of the batter into prepared pan and smooth with a spatula.
Slice 2 bananas into ¼" slices and layer on top of cake batter.
Pour second half of cake batter over bananas and smooth top with an offset spatula.
Sprinkle 2 tablespoons of granulated sugar evenly over top of cake.
2 Tablespoons Granulated Sugar
Bake for 50 - 60 minutes at 350℉ (180℃). The cake top will be a golden color and a toothpick inserted in the center of the cake will come out clean when the cake is done.
Place cake on a rack and allow to cool completely before unmolding.
If desired, sprinkle a few tablespoons of powdered sugar over top of cake after unmolding.