In a large Dutch oven or sauce pan heat butter and corn syrup over medium-high heat until combined. Add in the large marshmallows and heat until melted, stirring constantly.
Remove from the heat and add in the rice cereal, mix to combine. Fold in mini marshmallows. Grease a 9x13" pan.
Transfer the cereal mixture into the greased pan and spread out evenly, pressing down firmly. Let set for 20-30 minutes and slice.