Grandma Bonnie's Banana Bars

Preparation - Preheat the oven to 325°F. Spray a 13x18-inch (half sheet) rimmed baking sheet with nonstick cooking spray and line the bottom with parchment paper.

Make the batter - In a large bowl, mash the ripe bananas with a fork until mostly smooth (you should have about 1 cup mashed banana). Whisk the sugar and melted butter into the mashed bananas until combined. Sprinkle the baking soda and salt evenly over the banana mixture and whisk until fully incorporated, about 30 seconds. Whisk in the sour cream, eggs, and vanilla until combined. Using a rubber spatula, gently stir the flour into the banana mixture until just combined.

Bake the bars - Pour the batter into the prepared baking sheet and spread it into an even layer. Bake until the bars are just golden and a toothpick inserted in the center comes out clean, about 20 minutes, rotating the pan halfway through baking.

Bake the bars - Transfer the baking sheet to a wire rack and let cool completely, about 30 minutes.

Frost and serve - Spread the frosting evenly over the surface of the cooled bars. Sprinkle evenly with the toasted pecans or walnuts, if using. Cut into bars, wiping the knife clean between cuts as necessary, and serve. Store leftovers, covered, in the refrigerator for up to 3 days.