Preheat the oven to 350°F. Grease a 9x13-inch baking dish with non-stick cooking spray or butter, coating the bottom and sides. Set aside.

In a large mixing bowl, use an electric hand mixer or a stand mixer fitted with the paddle attachment to cream the softened butter and granulated sugar on medium speed until light, fluffy, and pale, about 2 to 3 minutes.

Add the eggs, sour cream, and vanilla extract. Beat on medium speed until fully combined and smooth, scraping down the sides of the bowl as needed.

Reduce the mixer speed to low and gradually add the flour, baking soda, and salt. Mix just until incorporated and a thick batter forms. Do not overmix.

Gently fold the mashed bananas into the batter with a spatula until evenly distributed.

Pour the batter into the prepared baking dish and spread it evenly. Bake for 25 to 35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. The top should be lightly golden brown.

Remove the cake from the oven and place the pan on a wire rack. Allow the cake to cool completely in the pan before frosting.

While the cake cools, beat the softened butter and cream cheese together on medium speed until smooth, creamy, and free of lumps. Add the vanilla extract and beat to combine. Gradually add the sifted powdered sugar, one cup at a time, beating on low speed until incorporated. Add the heavy whipping cream and beat on medium-high speed until the frosting is light, fluffy, and spreadable.

Spread the cream cheese frosting evenly over the completely cooled cake. Garnish with fresh banana slices and chopped walnuts or pecans just before serving, if desired.

Cut the cake into squares and serve. Store leftovers covered in the refrigerator for up to 5 days. Serve chilled or at room temperature.