Grease and line a 20 x 20cm (8 x 8") baking tin (see my notes).
Unwrap the Aero bars and trim to fit in the baking tin, then remove and set aside for later (see my notes).
In a large microwave safe bowl, combine peppermint filled chocolate (180g - broken into squares), salted butter (100g) and condensed milk (100g) and melt in 30 second bursts in the microwave. Mix to combine.
Add in crushed Choc Ripple biscuits (250g) and mix to combine, the press base mixture into greased and lined tin.
In a small microwave safe bowl place the milk chocolate melts (300g) and vegetable oil (2½ tbsp). Melt in the microwave in 30 second bursts (mixing between each burst) until smooth and melted.
Spoon 2-3 tablespoon of the melted milk chocolate over the base and spread to cover. Place the Aero bars evenly over the base.
Pour over the remaining melted milk chocolate, tilting to spread. Top with crushed Peppermint Crisp bar (if using) and refrigerate to set.