Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan, then line the bottom with parchment for easy lifting later. Quick tip: a light spray on top of the parchment helps the corners stick down.
Mash the bananas in a large mixing bowl until mostly smooth, a few soft bits are fine. Whisk in the melted butter, oil, granulated sugar, and brown sugar until glossy. Add the eggs, sour cream, and vanilla. Whisk again until the batter looks creamy and well combined. Take a breath. Smells good already.
In a separate bowl, whisk the flour, baking soda, baking powder, salt, and cinnamon. Add these dry ingredients to the wet ingredients. Switch to a spatula and fold gently until just combined. A few streaks of flour are okay. If using nuts, fold them in now. Resist the urge to keep stirring, because, well, we’re not trying to make a rubbery cake.
Pour the batter into the prepared pan and smooth the top. Tap the pan on the counter once or twice to pop any big air bubbles. Slide it into the oven.
Bake for 27–35 minutes, until the top is golden, springs back lightly, and a toothpick inserted in the center comes out clean or with a few moist crumbs. If your oven has hot spots (most do), rotate the pan at the 20-minute mark.
Let the cake cool in the pan on a rack for 20 minutes, then run a thin knife around the edges and lift out by the parchment if you like. Cool completely before frosting. I know it’s hard to wait, but warm cake will melt the frosting into a slip ’n slide.
Make the frosting: In a medium bowl, beat the softened cream cheese and butter with a hand mixer (or stand mixer) on medium speed until smooth and fluffy, about 2 minutes. Add the powdered sugar, vanilla, and salt. Mix on low until the sugar is incorporated, then increase to medium and beat until creamy, 1–2 minutes more. If it feels thick, add milk a teaspoon at a time until spreadable. If it feels too soft, add a few tablespoons more powdered sugar.
Frost the cooled cake. Swirl it on generously, edge to edge. If you’re feeling extra, scatter chopped nuts on top for crunch. Slice into squares and serve. Quiet moment, first bite. Yep, that’s the one.