Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 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, combine the mashed bananas, melted butter, granulated sugar, brown sugar, eggs, and 1 teaspoon vanilla extract. Whisk until well combined.
Step 3: In a separate medium bowl, whisk together the flour, baking soda, cinnamon, nutmeg, and 1/2 teaspoon salt.
Step 4: Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Begin and end with the dry ingredients. Mix until just combined. Do not overmix. If using, gently fold in chopped walnuts or pecans.
Step 5: Pour the batter into the prepared pan(s) and spread evenly.
Step 6: Bake for 30-35 minutes for round cakes, or 45-55 minutes for a 9x13 inch cake, or until a wooden skewer inserted into the center comes out clean.
Step 7: Remove from the oven and let cool in the pan for 10-15 minutes before inverting onto a wire rack to cool completely. Ensure the cake is fully cooled before frosting.
Step 8: For the frosting: In a large bowl, using an electric mixer, beat the softened cream cheese and softened butter together on medium speed until smooth and creamy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 9: Gradually add the sifted powdered sugar, one cup at a time, mixing on low speed until combined, then increasing to medium-high until light and fluffy (about 3-5 minutes).
Step 10: Beat in 1 teaspoon vanilla extract and a pinch of salt until just incorporated.
Step 11: Once the cake is completely cool, spread the cream cheese frosting evenly over the top using an offset spatula or knife. Slice and serve your Heavenly Banana Bread Cake Cream Cheese Frosting!