Combine cereal and nuts in a large bowl that has been sprayed with cooking spray.

Melt butter in a medium saucepan; add sugar and corn syrup.  Bring to a full rolling boil over medium heat, stirring constantly.  Continue to boil for 2 minutes.  Remove from heat; stir in vanilla and 1/2 cup chocolate chips.

Pour over cereal mixture; stir well to coat.  Toss with marshmallows. Pour onto greased or parchment lined baking sheet.  Spread mixture as much as possible. Press remaining 1/2 cup chocolate chips over top of snack mix. 

Cool for 1 hour or until chocolate is set.  Break into bite sized pieces.  Store in a tightly closed container.