In a medium saucepan, heat 2 cups of marshmallows, 1 cup of chocolate chips, 3 Tablespoons of butter. Stir occasionally, until completely melted. Remove from heat; stir in vanilla. Pour in cereal and mix until it is evenly coated. Stir in remaining 1 cup marshmallows and 1/2 cup of chocolate chips.
Press firmly in pan. Cool at least 1 hour until firm. For bars, cut into 3 rows by 3 rows. Store loosely covered at room temperature.