In a small/medium-sized saucepan, heat the butter over medium low heat until completely melted.
Add marshmallows and stir until completely melted.
Stir in cereal until combined.
Pour cereal mixture onto prepared cookie sheet and spread thin (it's fine if it's not a solid sheet and is stringy).
Allow the rice krispies to cool completely.
Once Rice Krispies have cooled, break into small pieces (think: the size of a cookie dough bite in cookie dough ice cream, or smaller) and then prepare your ice cream.
Ice Cream
In a medium-sized bowl, stir together condensed milk and marshmallow fluff until completely combined.
Add milk and vanilla extract and stir until combined.
In a separate bowl, using a hand mixer or stand mixer with whisk attachment, beat the heavy cream
until stiff peaks form.
Fold the marshmallow/condensed milk mixture into the heavy cream until completely combined (be sure to scrape the sides and bottom of the bowl).
Take about ¾ of your rice krispie pieces (it doesn't have to be exact) and stir into the ice cream mixture.
Pour into a 9x5 bread pan (I line mine with wax paper or parchment paper to protect my nonstick coating from the ice cream scoop) and sprinkle with the remaining rice krispie pieces.