In a small/medium-sized saucepan, heat the butter over medium low heat until completely melted.
2 Tbsp Butter
Add marshmallows and stir until completely melted.
12 Marshmallows
Stir in cereal until combined.
2 cups crisped rice cereal
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.
1 can sweetened condensed milk, 7 oz marshmallow fluff
Add milk and vanilla extract and stir until combined.
½ cup milk, ½ tsp vanilla extract
In a separate bowl, using a hand mixer or stand mixer with whisk attachment, beat the heavy cream
2 cups 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.