Grease a 9x13-inch pan with butter or nonstick spray.
Place the butter, 5 cups of miniature marshmallows, and vanilla extract into a large stock pan. Melt over medium-low heat, stirring frequently, until melted.Alternatively, place in a large microwave-safe bowl and microwave in 30-second intervals, stirring in between, until melted. If the marshmallows are not yet melted after 1 minute, continue microwaving in 10-second intervals.
Pour the cereal into the melted marshmallow mixture. Stir until the cereal is completely coated with the marshmallows. Set aside for 6-8 minutes, until the marshmallows are cool to touch.
Gently stir in the remaining marshmallows and ¾ cup of chocolate chips until they are evenly distributed throughout the cereal mixture.
Transfer the cereal mixture into the 9x13-inch pan. Using your spatula, gently press into the bottom of the pan, being careful not to pack in too firmly as that will make the bars stiff.
Sprinkle remaining chocolate chips over the top of the bars. Cool completely.
Spray the spoon or spatula with nonstick spray before stirring the cereal into the marshmallows. This will help prevent the marshmallow from sticking.
Work quickly when stirring in the chocolate chips so that they can be mixed in thoroughly before they start to melt.
For clean slicing, spray a knife with nonstick spray before cutting the bars.
Store in an airtight container at room temperature, for up to 5 days.