In large saucepan over very low heat, stir chocolate chips and peanut butter until melted and smooth. (or microwave at HIGH 3 minutes in large microwaveable bowl, stirring twice).
Remove from heat.
Stir in vanilla.
Add cereal and marshmallows, mixing until well coated.
Press evenly and firmly into buttered 3.5 L (13 x 9") pan.
Chill until firm, about 45 minutes. Cut into bars.
Variation 1:
Frosted: Spread 250 ml (1 cup) melted semi-sweet chocolate chips over cereal mixture.
Variation 2:
Crunchy: Add 125 ml (½ cup) chopped peanuts with cereal.