Mix the melted butter, caster sugar and brown sugar together in a large bowl. Add the egg, vanilla extract and peppermint extract and mix well until smooth.
Stir in the self-raising flour and cocoa powder. You should end up with a soft, sticky dough.
Stir/press in the chocolate chips and Mint Aero chocolate.
Divide the dough into 12 equal-sized pieces and roll each piece into a ball.
Place the cookie dough balls onto a lined baking tray, leaving a bit of space between them, then freeze for 1 hour.
Preheat your oven to 180°C (160°C for fan ovens).
Bake the cookies straight from the freezer for around 15 minutes. They should be slightly firm around the edges but soft in the middle (they'll firm up as they cool).
Let the cookies cool completely on the tray before enjoying.