Butter a 9x13-inch baking dish or line with parchment paper; set aside
Grease the inside of a large mixing bowl with butter, then pour in the cereal and the mini Rolos. Reserve some Rolos to top the bars if desired.
Melt the butter in a large saucepan on low heat. Add the bag of marshmallows. Stir, melting the marshmallows, until they have dissolved into a smooth mixture. Remove from the heat and stir in the vanilla extract.
Let set and then slice to serve!
Pour the marshmallow mixture over the cereal and gently stir to coat evenly. Turn out the cereal mixture into the prepared pan. Sprinkle the reserved Rolos on top.
Butter your hands, then firmly press the mixture into an even layer. Cover with plastic wrap and let the pan sit until it sets, about an hour.
Slice into squares and serve. Store in a closed container for up to 5 days at room temperature.