Line a 9x13 baking dish with parchment paper or grease with softened butter.
Add the Rice Krispies cereal into a large mixing bowl. Set aside.
In a large heavy heavy-bottomed pan on medium heat. Add the butter and allow it to fully melt.
Pour into the condensed milk and mix well. Reduce the heat to low-medium.
Add the mini marshmallows, about 1 ½ bags (save the remaining to mix in later). Mix well until fully melted.
Remove from the heat and add the vanilla extract. Mix well.
Carefully pour the marshmallow mixture into the mixing bowl with the cereal.
With a rubber spatula, mix the cereal into the marshmallow mixture until fully coated. Add the remaining half of the mini marshmallows. Mix well.
Scrape the mixture into the prepped baking dish. Gently press down and push the mixture into a single layer. Try not to overpress or pack down the mixture. Can top with extra marshmallows.
Let fully cool and step up for a minimum of 1 hour. Cut into squares and enjoy.