Step 1

Preheat oven to 350°F. Line a 15x10x1-inch baking pan (jelly roll pan) with parchment paper and spray with nonstick cooking spray.

Step 2

In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, cream together the butter and sugars until light and fluffy; about 3 minutes. Add eggs one at a time to the creamed mixture. Mix in sour cream and vanilla until fully incorporated into the batter.

Step 3

In a medium bowl, sift together the flour, baking soda and salt. Gradually add the dry ingredients to the butter and sugar mixture and mix until combined. Stir in bananas and walnuts or pecans until they are fully incorporated into the batter.

Step 4

Evenly spread batter into prepared pan. Bake in preheated over for 25-30 minutes or until a toothpick inserted into the center of the bars comes out clean. Remove pan to a wire rack to cool completely.

Step 5

While the bars are cooling, prepare the cream cheese frosting. In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, beat the cream cheese and butter on medium-high speed for about 3 minutes. Mix in the vanilla. With the mixer on low, gradually add in the confectioners' sugar and mix until fully combined. Turn mixer up to medium-high speed and beat until super fluffy; about 3-4 minutes. Use an offset spatula to frost the bars or pipe the frosting onto the bars (see note below).