Preheat your oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Grease the brownie tin and line it with tin foil, including the sides of the tin.
Mix the egg, water, oil and brownie mix together until well blended (mixture may be lumpy).
Spread the mixture evenly into the lightly-greased rectangular tin.
Bake in the centre of the oven for around 20-25 minutes or until a rounded knife, inserted 5cm (2") from the edge of the tin comes out almost clean. Allow to cool in the tin for at least an hour.
Combine mini-marshmallows, chocolate chunks, peanut butter and butter into the saucepan and stir over gentle heat until all the ingredients are well combined. Gently fold in the rice puffs.
Pour the mixture on top of the cooled brownie in the tin and spread out evenly. Pop in the fridge for at least an hour.
Cut the brownie mix into 16 pieces with a sharp knife before serving.
Enjoy a delicious slice… mmm!