Stir together the chex and peanuts in a large bowl. Line a 9x13 inch pan with foil or parchment paper. Spray with cooking spray or spread with softened butter.

In a large pot, bring corn syrup, peanut butter, and sugar to a boil over medium heat. Boil for 45-60 seconds. Don't boil too long, or your bars will be dry and hard to chew!

Remove from the heat and stir in the vanilla.

Pour the peanut butter mixture over the cereal. Stir until evenly coated.

Dump the mixture into the prepared pan and press into an even layer with wet hands.

Use the foil to lift the bars onto a cutting board. Use a sharp knife to cut into squares.