Line a 20cm/8inch square tin with baking parchment.
Place the butter, drinking chocolate, condensed milk and 1 of the Aero bars in a a small saucepan and melt over a gentle heat. Leave to cool a little, so it doesn’t melt the chocolate at the next stage.
Crush the biscuits, trying to leave a few larger lumps for texture. Chop 2 of the Aero bars into small pieces.
Pour the cooled buttery, chocolatey mixture over the biscuits and chopped Aero and mix well. Press into your traybake tin and set aside or refrigerate until set.
Melt the white chocolate in a small bowl over a pan of simmering water and then spread over the base. Top with the final, chopped, Aero bar.