Preheat oven to 325°F. Generously grease two 9-inch round cake pans. Line the bottom with a circle of parchment paper (see note 3). Grease the parchment paper and lightly dust the entire cake pan with flour.
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 1-1/2 cups mashed bananas and then toss with lemon juice to prevent browning.
In the bowl of a stand mixer fitted with the whisk attachment, cream together the butter, granulated 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.
In a separate bowl, add in the spooned and leveled flour, baking powder, baking soda, salt, and cinnamon. Place the mini chocolate chips in a bowl and add a bit of flour, shake them around the bowl until they are coated in flour. Add the chocolate chips to the other dry ingredients, stir together to combine.
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, be 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.
Divide the batter evenly between the two prepared cake pans. Bake in the preheated oven for 45–50 minutes, or 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, then invert the cakes onto a wire cooling rack to finish cooling completely. Do not frost until fully cooled.
To make the frosting, use a hand-held or stand mixer fitted with a paddle attachment, beat the butter on medium speed for 3–4 minutes. Add the powdered sugar and cocoa powder and mix on low. Add heavy cream and 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 the salt. If the frosting is still too sweet, add a dash more salt.
If the cakes are 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 or plate. 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. Frost the top and sides of the cake. If desired, decorate with additional mini chocolate chips. Slice and serve.
Notes
Note 1: Make sure to spoon and level the flour measurement. If you scoop a measuring cup directly into a bag of flour, you will pack in too much flour which creates a drier and less-flavorful cake.Note 2: For best flavor and texture, use Dutch-process, not natural cocoa powder, for the frosting.Note 3: Make a parchment paper round bottom for the cake pans by following these steps:
Tear off a sheet of parchment paper slightly larger than the cake pan.
Fold the parchment paper in half, then fold in half again.
Fold into a triangle, then fold the triangle in half.
Hold the triangle against the bottom of the pan from the center outwards.
Cut the parchment paper along the curve, right where it meets the edge of the cake pan.
Unfold the parchment paper and press into the bottom of the cake pan.
Attribution: This Banana Cake recipe was adapted, with permission, from Sally’s Baking Addiction Cookbook.Nutrition Note: Nutrition information includes the frosting.Storage: Keep the cake in an airtight container, unrefrigerated, for up to 3 days. Store in an airtight container in the fridge for up to a week. Wrap slices individually and freeze for you to 3 months, then thaw at room temperature.