01 -
Preheat oven to 175°C (350°F). Prepare a 23x33cm (9x13-inch) baking dish by coating with non-stick cooking spray. Set aside.
02 -
In a large bowl with an electric hand mixer, or in a stand mixer fitted with the paddle attachment, cream butter and sugar together until light and fluffy (about 2-3 minutes). Initially the mixture will appear crumbly but will become fluffy with continued beating.
03 -
Incorporate eggs, sour cream, and vanilla extract. Blend until thoroughly combined and creamy in texture.
04 -
Add flour, baking soda, and salt. Mix on low speed just until incorporated. Fold in the mashed bananas gently until evenly distributed throughout the batter.
05 -
Transfer batter to the prepared baking dish, spreading evenly. Bake for 25-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs or clean.
06 -
In a large bowl, beat butter and cream cheese together until smooth and well combined. Add vanilla extract, powdered sugar, and heavy cream. Begin beating at low speed, gradually increasing as needed, until a smooth, creamy frosting forms.
07 -
Once the cake has completely cooled to room temperature, spread the cream cheese frosting evenly over the top. Cut into squares and garnish individual servings with banana slices and chopped walnuts if desired.