Preheat the oven to 350°F. Grease and flour two 9" round baking pans (or spray with a flour-coating baking spray).

In a large bowl, whisk together the 115 oz. boxchocolate cake mix and 1 tsp cinnamon.

In a separate bowl, whisk together 1 cup buttermilk (or homemade buttermilk substitute), 3 eggs, 1 tbsp vegetable oil, and 1 tsp vanilla extract until combined.

Pour the wet ingredients into the dry ingredients and stir until mostly combined. Fold in 1 1/2 cups mashed bananas (about 3 ripe bananas) until batter is uniform; do not overmix.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 35–40 minutes, or until a toothpick inserted into the centers comes out clean.

Remove the cakes from the oven and let them cool in the pans for about 10 minutes. Then invert the cakes onto a wire rack and cool completely before frosting.

While the cakes cool, make the frosting: in a large bowl or the bowl of a stand mixer, beat 1/2 cup salted butter (1 stick), 1 tsp salt, 1 tsp lemon juice, and 1/2 cup mashed ripe banana together on medium speed until combined and smooth.

With the mixer on low, add 3 cups powdered sugar, 1/3 cup cocoa powder, and 1 tsp espresso powder (optional). Add 2 tbsp heavy cream or whole milk and mix until the powdered sugar and cocoa are incorporated. If the frosting is still too dry or not coming together, add the remaining 1 tbsp (use a total of 2–3 tbsp as needed).

Increase the mixer speed to high and beat the frosting for 5 minutes, scraping down the bowl once or twice. The frosting should be light and fluffy.

To assemble: place one cooled cake layer on a serving plate. Spread about one-third of the frosting evenly over the top of that layer.

Place the second cake layer on top, then use the remaining frosting to frost the top and sides of the assembled cake.

Chill the frosted cake in the refrigerator or freezer for about 15 minutes if you want the frosting to set (optional). Slice and serve.