Place butter and brown sugar into a medium saucepot over medium high heat. Stir from time to time and bring just to a boil. Stir in corn syrup, salt, vanilla, and baking soda. It will foam up a bit. Remove from heat and set aside.
Stir together cereals and nuts (if using) in a large bowl or dishpan. Pour syrup over and stir well to coat evenly.
Pour this in batches into a large brown paper grocery bag, roll down the top to close it, and microwave for two minutes. Take bag out and shake. Return to microwave for another two minutes and then shake again. (Be careful to hold it by the closed top when you shake it to keep it closed and prevent anything from flying out. This will because that is how microwaves work).
Carefully pour hot chex mix out into waxed paper and allow to cool and dry. Repeat with any remaining chex mix. Enjoy!