1. Spray 9 x 9 pan.
2. Heat corn syrup and butter to boiling in a 3 quart saucepan, remove from heat.
3. Add chocolate chips and vanilla. Stir until chocolate is melted.
4. Fold in crumbs, 1 cup at a time, stirring until completely coated. Fold in marshmallows.
5. Press mixture evenly in pan with buttered spoon.
6. Let stand until firm (1 hour). Cut into bars.