PREP: Preheat oven to 325 degrees F. Generously grease two 9-inch round cake pans. Line the bottom with a circle of parchment paper (essential to get these cakes out of the pan nicely!). See Note 3. Grease the parchment paper and lightly dust the entire cake pan with flour.
BANANAS: Thoroughly mash the bananas so no chunks remain; I like to mash them quickly by beating with a hand mixer. Measure to get a level 1 and 1/2 cups mashed bananas and then toss with lemon juice to prevent browning.
WET INGREDIENTS: In the bowl of a stand mixer fitted with the whisk attachment, cream together the butter, white sugar, and dark brown sugar on medium-high speed until light and fluffy, about 2-3 minutes. Add in the sour cream, vanilla, and 1 large egg. Beat to combine. Add the next egg; beat to combine. Add the final egg and beat. Stir in the mashed bananas.
DRY INGREDIENTS: In a separate bowl, add in the spooned and leveled white flour, baking powder, baking soda, salt, cinnamon, and miniature chocolate chips. Stir together to combine, making sure the chocolate chips are coated in flour to keep them from sinking in the batter.
BUTTERMILK AND DRY INGREDIENTS: With the mixer set on low speed, slowly add 1/3 of the flour mixture and then 1/3 of the buttermilk. Mix until just combined, being careful to not over mix. Repeat with the second and third portions of the flour and buttermilk. Mix until just incorporated. Stop mixing as soon as ingredients are combined.
BAKE: Divide the batter evenly between the two prepared cake pans. Bake in the preheated oven for 45-50 minutes, until a toothpick inserted into the center comes out clean or with just a few moist crumbs. Allow to cool for 15 minutes in the cake pans and then invert the cakes onto a wire cooling rack to finish cooling completely. Do not frost until fully cooled.
FROSTING: Using a hand-held or stand mixer fitted with a paddle attachment, beat the butter on medium speed for 3-4 minutes. Add 3 cups of powdered sugar and cocoa powder and mix on low. Add 3 tablespoons of heavy cream and the vanilla extract. If the frosting is too thin, add more powdered sugar (up to 1 cup) until you get the desired consistency. If the frosting is too thick, you can add more heavy cream (up to 2 tablespoons). Add in the salt. If the frosting is still too sweet, add a dash more salt.
ASSEMBLE: If the cake layers are slightly domed on top, carefully level them off with a sharp serrated knife or cake leveler. To build the cake, smear a tiny bit of frosting in the center of your cake stand/plate (to keep the cake from sliding). Next, place the first layer bottom side up. Using a knife or offset spatula, spread the top of the cake evenly with the chocolate frosting. Place the second layer on top, top side up, and spread more frosting evenly over the top. Frost the top and sides of the cake. Decorate with additional miniature chocolate chips, if desired. Slice and serve. This cake is best enjoyed within 2-3 days.