Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9×13 inch baking pan or two 9-inch round cake pans. If using round pans, line the bottoms with parchment paper.
Step 2: In a large bowl, mash the very ripe bananas thoroughly with a fork or potato masher until mostly smooth with a few small lumps remaining.
Step 3: Add the melted butter, granulated sugar, brown sugar, eggs, and 1 teaspoon vanilla extract to the mashed bananas. Whisk until well combined.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt until thoroughly combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined. Be careful not to overmix.
Step 6: Pour the batter evenly into your prepared baking pan(s).
Step 7: Bake for 30-35 minutes for round cakes, or 40-50 minutes for a 9×13 inch pan, or until a wooden skewer inserted into the center comes out clean.
Step 8: Let the cake cool in the pan(s) on a wire rack for 10-15 minutes, then invert onto the wire rack to cool completely. The cake must be completely cool before frosting.
Step 9: Prepare the cream cheese frosting: In a large bowl, using an electric mixer, beat the softened cream cheese and softened unsalted butter together on medium speed until smooth and creamy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 10: Add 1 teaspoon vanilla extract and beat until just combined.
Step 11: Gradually add the sifted powdered sugar, one cup at a time, beating on low speed until incorporated, then increasing to medium speed until smooth and fluffy. If the frosting is too thick, add milk or cream one tablespoon at a time until desired consistency is reached.
Step 12: Once the banana bread cake is completely cool, spread the cream cheese frosting evenly over the top. Serve and enjoy!