Butter 8- or 9-inch square baking pan; set aside.
Combine 1/4 cup butter and marshmallows in 4-quart nonstick saucepan. Cook over medium-low heat, stirring constantly, until mixture is melted and smooth. Remove from heat. Stir in cereal, decorator sprinkles and vanilla.
Spoon mixture into prepared baking pan. Press down firmly with buttered hands. Cool completely.
Meanwhile, combine all drizzle ingredients in small bowl; beat at low speed until creamy. Drizzle over cooled bars. Cut into bars.
Use marshmallows that are soft and fresh as they melt more evenly and are less sticky.