Butter square pan, 8x8x2 or 9x9x2”.
Heat butterscotch chips, corn syrup, margarine and vanilla in 3-quart saucepan over low heat, stirring constantly, until butterscotch chips are melted and mixture is smooth.
Remove from heat.
Stir in cereal and miniature marshmallows until well coated.
Press evenly in pan, using buttered back of spoon; cool