In a saucepan over medium heat, melt the butter. Once melted, add the mini marshmallows and stir until completely melted and smooth. Remove from heat.
In a separate bowl, whisk together the instant banana pudding mix and milk until well combined. Let it sit for about 2 minutes to thicken.
Pour the melted marshmallow mixture into the Rice Krispies cereal in a large mixing bowl, and stir until evenly coated.
Add the banana pudding mixture, vanilla extract, and sliced bananas to the Rice Krispies. Gently fold everything together until combined.
Pour the mixture into a greased 9x13-inch baking pan, pressing down firmly to create an even layer.
Allow the mixture to cool for at least 30 minutes at room temperature or in the fridge to set.
Once cooled, cut into squares and serve. If desired, top with whipped cream or Cool Whip before serving.