In a large pot over low heat, melt the butter. Add mini marshmallows and stir until smooth and glossy.
Whisk in the dry banana pudding mix and milk. Stir until creamy and fully combined.
Remove from heat and gently fold in the Rice Krispies cereal until evenly coated.
Press the mixture into a parchment-lined 9x13-inch pan using a greased spatula. Do not compact too tightly.
Sprinkle crushed vanilla wafers over the top. Chill for 20 minutes before cutting into squares.
Optional: Top with sliced bananas just before serving.