Butter bottom and sides of a 13 x 9 inch baking pan with 1 Tbsp softened butter or spray with cooking spray. Set aside.
In a deep pot on the stovetop over low heat, melt together butter, 10.5 oz mini marshmallows, light corn syrup and 1 1/2 cup milk chocolate chips. Melt for 3-5 minutes stirring constantly until completely smooth. Add vanilla, mixing well.
To the mixture add cereal, 1 cup mini marshmallows and remaining 1 cup milk chocolate chips. Work quickly, stirring until the cereal is completely coated.
Press the mixture into the buttered pan. Gently press the remaining 3/4 cup mini marshmallows on top. Cool for at least 1 hour or until set.
Cut using a sharp knife into bars. Store in an airtight container at room temperature for up to 1 week.