Got one overripe banana sitting on your counter? These fudgy banana brownies are exactly what it was made for! Sweetened condensed milk baked right into the batter keeps them moist and gooey.
Grease an 8 by 8 square pan with butter. Line all sides with parchment paper. Leave an overhang on at least two sides. Butter the parchment paper. Set aside. Preheat oven to 350℉.
To prepare the (optional) Nilla wafers, place the wafers in a zip-top bag, seal it, and gently crush them with a rolling pin until they are broken into a mix of mostly pea-sized pieces with some smaller crumbs and a few slightly larger chunks.
In a medium saucepan, melt the butter and sweetened condensed milk over medium heat. Stir often until butter has melted. Remove from heat.
Add the chopped semisweet chocolate to the melted butter mixture. Stir until chocolate has melted. Remove from heat and let cool for about 10 minutes. (The mixture should be warm but not hot before combining with the eggs).
In the bowl of an electric mixer or a large mixing bowl using a handheld mixer, beat the sugars, eggs, egg yolk and vanilla extract on medium high for 3 minutes.
Pour the warm chocolate mixture into the egg mixture in a slow, steady stream, stirring constantly with a rubber spatula as you pour. Add the mashed banana and stir to combine.
Sift in the cocoa powder, salt and flour. Add the chocolate chips. Stir with a spatula until just combined. Avoid overmixing.
Pour mixture into the prepared pan. Bake 38-44 minutes until a thin knife inserted in the middle of the brownies comes out with moist fudgy crumbs (not wet batter). Brownies are done when the edges look set and begin to pull slightly away from the pan, and the center is set but still very fudgy and soft rather than liquid. The brownies will continue to set as they cool.
Place pan on a cooling rack and cool completely. Allow the brownies to cool completely in the pan for at least 2 to 3 hours before slicing.
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Notes
Semisweet chocolate: You can find bars of semisweet chocolate in the baking aisle of grocery stores. Sweetness level: I don't recommend reducing the amount of sugar since that would affect the texture of the brownies. To reduce the sweetness, swap in unsweetened chocolate in place of semisweet chocolate. Storage: Store leftover brownies in an airtight container at room temperature for up to 4 days.