Melt butter in a large pot over medium-low heat. It will get foamy, turn clear and then golden brown and begin to smell nutty. Remove from heat. Keep a close eye on it after the foam disappears, as it will burn quickly.
2
Add marshmallows to butter and stir until melted. You may need to return the pot to the stove on low heat to completely melt the marshmallows, but if you allow them to cook too long, the sugars will caramelize and the bars will be hard and dry when they cool.
3
Add salt and vanilla and begin to stir in.
4
Add rice crispy cereal slowly and stir until well mixed in. Press mixture into buttered pan using a silicone spatula or greased wax paper and allow to cool. Cut into squares or bars.
5
(recipe slightly adapted from www.smittenkitchen.com)