1. Preheat and Prep: Set oven to 350°F (177°C). Grease a 9×13-inch pan, or two 8-inch round pans for layers, or a bundt pan. Line with parchment for easy release.

2. Mash Bananas: Peel and mash bananas with a fork until smooth with some chunks for texture. Set aside.

3. Mix Dry Ingredients: Whisk flour, baking soda, cinnamon, and salt in a medium bowl.

4. Cream Butter and Sugars: Beat butter, granulated sugar, and brown sugar on medium-high until pale and fluffy, about 3 minutes. Add eggs one at a time, beating well after each. Mix in vanilla.

5. Combine Wet and Dry: Add mashed bananas to the butter mixture. On low speed, alternate adding dry ingredients and sour cream/buttermilk (start and end with dry). Mix just until combined, about 30 seconds. Fold in chopped walnuts gently to avoid toughness.

6. Bake: Pour batter evenly into pan(s). For 9×13, bake 35-40 minutes; 8-inch layers 25-30 minutes; bundt 50-60 minutes. Toothpick in center should come out clean or with moist crumbs. Cool in pan 10 minutes, then fully on wire rack.

7. Make Frosting: Beat cream cheese and butter until smooth. Gradually add powdered sugar, vanilla, and salt. Beat on high until fluffy, 2 minutes.

8. Frost and Serve: For sheet cake, spread frosting evenly and sprinkle walnuts. For layers, level cakes, add crumb coat, chill 30 minutes, then frost fully. Garnish with walnuts.