Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease and line a 20cm square cake tin with butter and baking paper.
Stir the brownie mix, butter and eggs with a wooden spoon until combined. Spoon the mixture into the prepared pan and spread evenly over the base.
Bake for 20-25 minutes then remove from the oven, transfer to a cooling rack and leave to cool completely, about 1 hour.
Spread the vanilla icing over the brownie, then sprinkle with the peanuts and chill in the fridge whilst you prepare the topping.
Microwave the chocolate chips and peanut butter uncovered on high for 1-2 minutes, stirring every 30 seconds, until melted. Stir in cereal and mix together until all combined and covered in the chocolate mixture. Spoon onto the vanilla iced brownie then cool and chill in the fridge for 1 hour, until cutting.
Cut into 4 rows by 4 rows. Store tightly covered at room temperature.