Step 1

Preheat oven to 350°F. Line a 9×9 inch square baking pan with parchment paper, leaving the edges hanging off of the side (to lift the cake out when it is done).

Step 2

In a medium bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and brown sugar. Set aside.

Step 3

In a separate bowl, combine bananas, oil, sour cream, and vanilla. Stir in the eggs until everything is well combined.

Step 4

Add the flour mixture to the banana mixture and stir until just barely combined (do not overmix). Pour the batter into the prepared baking pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Step 5

Cool on a wire rack completely before frosting.

Step 6

In a medium saucepan over low heat, melt the chopped chocolate until smooth. Once melted, remove from the heat to cool slightly.

Step 7

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, beat the butter on high speed until light and fluffy (about 3 minutes).

Step 8

Add the vanilla extract and salt and beat on medium speed until incorporated.

Step 9

Reduce the mixer to low speed and slowly add in the powdered sugar. Beat on low for 30 seconds and then increase the speed to medium-high and beat until smooth (about 3 minutes).

Step 10

Stop the mixer and add the melted chocolate to the bowl. Mix on low speed until combined. Frost cooled cake with buttercream.