Preheat your oven to 180°C (160°C for fan assisted ovens) / Gas Mark 4.
Mix the egg, water, oil and brownie mix together until well blended (mixture may be lumpy).
Spread the mixture evenly into the well-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. Leave to cool.
Mix the softened butter, peanut butter, icing sugar and milk (by hand or electric whisk) for 2-3 minutes until smooth. Spread the mixture evenly over the brownie base.
Melt the butter and chocolate chips, in your small microwaveable bowl, in the microwave for 30-60 seconds. Stir until smooth and cool for 10 minutes.
Then spread this over the peanut butter filling. Refrigerate for 30 minutes or until set and then cut into squares… Simply divine!