Mix butter, brown sugar, and Karo syrup together and microwave on high to boiling.
Add the baking soda and vanilla and stir well.
In a large brown paper grocery bag, put in the Crispix cereal and pour the syrup mixture over it and stir well.
Microwave 1 minute.
Shake bag well, or stir to mix with a long wooden spoon-careful not to burn your hand or arm while sticking it in the bag.
Repeat 3 times microwaving for 1 minute and stirring after each minute.
Spread mixture out on wax paper to cool.
Keep mixing it often to keep it from sticking together in big clumps.