Prepare your baking dish by spraying it with cooking spray or lining it with parchment paper.
In a medium saucepan, melt the butter over low heat. Once melted, add mini marshmallows and stir until completely melted and smooth.
Remove the saucepan from heat. Add the banana pudding mix and vanilla extract, stirring until well combined.
Gradually add the Rice Krispies cereal to the marshmallow mixture, stirring until the cereal is evenly coated.
Gently fold in the sliced bananas, being careful not to mash them.
Pour the mixture into the prepared baking dish. Press it down evenly with a spatula or your hands (lightly greased to avoid sticking).
Allow the treats to cool at room temperature for about 10 minutes, then refrigerate for an additional 15 minutes to set.
Once chilled, cut into squares and serve. Optionally, top with whipped topping just before serving.