Prepare a 9-inch by 9-inch baking pan by greasing it with oil or butter, then lining it with parchment paper.

Preheat the oven to 350ºF.

In a large mixing bowl, mash the three bananas, then add the melted butter and brown sugar. Mix until light and fluffy.

Add the egg and vanilla extract, and mix until fully incorporated.

Sift in the flour, baking powder, and salt.

Mix the batter just until combined, without overmixing.

Pour the batter into the prepared baking pan.

Bake for 25–30 minutes, until a toothpick inserted into the blondies comes out clean.

Cool in the pan for 15–30 minutes, then remove and transfer to a wire rack to cool completely.

Add the chocolate chips and peanut butter to a bowl.

Microwave in 30-second increments, stirring between each, until melted and smooth.

Add the icing sugar and mix until it comes together.

Assembly

Transfer the cooled blondies back into the baking pan.

Pour the peanut butter chocolate frosting on top of the banana blondies.

Using an offset spatula, spread the frosting into an even layer on top of the blondies.

Cover the baking dish with plastic wrap and transfer to the fridge to let the frosting set and firm up.

Once chilled, cut the blondies into 12–16 even squares to serve.