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.