Melt together the butter, sugar, brown sugar, and marshmallows in a large saucepan over medium to medium low (DO NOT DO TOO HIGH HEAT, otherwise it’ll go hard). See notes.
Bring to a small simmer, and simmer for 3 minutes (not a full rolling boil)! You will see small bubbles bursting on the surface of the mixture.
Remove from heat and stir in the vanilla and salt. Next, add the 5 cups of Special K cereal into the mixture. Stir to combine, then spread out on a cookie sheet to set.
Store in an airtight container or plastic bag.
Notes
When melting the butter, sugars, and marshmallows be aware that it will take some time. Mine took about 20 minutes or so to melt down. Try and be patient with it.