Preheat your oven to 180C/160C/Gas mark 4 and line a 9″ square baking tray with parchment paper.
Melt together the Butter, and Dark Chocolate in a heatproof bowl over a pan of boiling water. Leave to the side to cool to room temperature.
Using an electric whisk, whisk together the eggs and caster sugar for a few minutes until the colour has turned pale, is very mousse like, and is double the original volume of the amount of eggs + sugar. You will know its done because when you lift the whisk up out of the mix it should leave a trail for a couple of seconds before disappearing.
When whisked, pour the cooled chocolate mix over eggs, add the peppermint essence and fold together carefully. Don't fold it to quickly as you don’t want to knock out the air.
Once completely combined, sift the cocoa powder and plain flour on top of the chocolate mix and then fold together again.
Once combined, fold through the Mint Aero chocolate and mint matchsticks and pour into the prepared tin. Bake in the oven for 25-30 minutes! Once baked, leave to cool in the tin.