Preheat the oven to 350°F. Line a 9x13 baking dish with parchment paper, set aside.
In a medium sized bowl, stir together the flour, baking powder, and salt, set aside.
Place the melted butter into a large bowl with the brown sugar, mashed banana, egg, and vanilla, whisk until smooth.
Add the flour mixture and stir until combined with no dry patches.
Spread the batter into the prepared baking dish. Bake for 30-35 minutes until the edges are golden brown and a toothpick inserted into the center comes out clean.
Place the baking dish on a wire rack to cool completely.
While the blondies are cooling, make the frosting by placing the cream cheese and butter into a large bowl and whip it until smooth with an electric hand mixer. You can also do this with a stand mixer with the paddle attachment.
Add the powdered sugar a little at a time until combined, scrape down the sides as needed. Add the vanilla, turn the mixer on to medium speed and whip for 3 minutes until light and fluffy.
Lift the blondies out of the baking dish using the parchment paper to help. Frost them and cut into bars, serve immediately.
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Notes
If you are making the cream cheese frosting, these will need to be refrigerated. If you are not adding cream cheese frosting, these can stay at room temperature for up to 3 days in an airtight container. Freeze for up to 3 months.