Preheat oven to 325F if using grape nectar, 350F if using regular sugar
In medium mixing bowl, use fork to mash bananas. Add either 1) grape nectar + granulated sugar OR 2) just regular sugar. Add coconut oil and vanilla. Whisk to combine.
Add flour, salt, and cocoa powder. If using grape nectar, add baking soda as well. Whisk to combine.
Grease 9x9 pan and pour in batter. Spread evenly with spatula; don't worry if the layer of batter seems too thin.
Add spoonfuls of peanut butter spaced 1-2 inches apart on the top of the batter. Use a butter knife to swirl the peanut butter through the batter.
Bake for 25 minutes on the center rack of the oven until the center is set and no longer jiggles when the pan is shaken.
Cool completely before cutting and serving.