Peanut Butter-Marshmallow Treats

32 large marshmallows Or
3 cups miniature marshmallows
1/4 cup butter or margarine
1/2 teaspoon vanilla
5 cups Reese's Peanut Butter Puffs cereal
Spray square pan, 9 9 2 inches, with cooking spray. Heat marshmallows and butter in 3-quart saucepan over low heat, stirring constantly, until marshmallows are melted and mixture is smooth; remove from heat.
Stir in vanilla.
Stir in half of the cereal at a time until evenly coated. Press in pan; cool. Cut into 6 rows by 6 rows.