Prepare the brownie mix according to the box instructions. (Be sure that you're preparing it in a 9x13 pan - some brownie mixes these days only fit an 8x8 square pan).
Remove the brownies from the oven and immediately put sprinkle the marshmallows over-top; return to the oven for 3 minutes.
While the brownies cool a bit, make the topping. In a microwave-safe bowl, stir together the chocolate chips, corn syrup, butter.
Microwave in 30 second intervals, stirring after each one, until smooth.
Add the peanuts and Rice Krispies and stir until they are evenly coated.
Carefully spoon the mixture over the marshmallow-covered brownies. Use a rubber spatula or knife to spread it out evenly.
Cool the bars completely before cutting. (You can put them in the fridge to help set the bars more quickly, but cut them before they harden too much!)