Heat oven to 350°F. Line 8-inch square pan with foil or cooking parchment paper; spray foil with cooking spray.
In bowl of stand mixer with paddle attachment, place 1 ripe medium banana and 1 roll refrigerated sugar cookies. (You can also use an electric hand mixer or a wooden spoon.) Beat on low to medium-low speed until smooth. Press mixture in bottom of pan.
Bake 35 to 40 minutes or until toothpick inserted in center comes out clean. Cool completely, at least 30 minutes.
To make frosting, beat 4 oz cream cheese, softened, and 2 tablespoons butter, softened, with electric mixer until smooth. Gradually add 2 cups powdered sugar, 1 cup at a time, until smooth. Beat in 1/2 teaspoon vanilla and 1/2 teaspoon whipping cream or milk, adding more as needed for spreading consistency.
Spread frosting over cooled bars. Cut into 6 rows by 4 rows. Store in airtight container in refrigerator up to 3 days.