Preheat the oven to 170 degrees, 325F, gas mark 3. Line a brownie tin with greaseproof paper.
Put the eggs and sugar in a bowl and use an electric whisk to whisk together until the mixture is pale and has doubled.
Put the chocolate and butter in a heatproof bowl over a saucepan of simmering water leave until all melted and leave to cool.
Once the chocolate mixture has cooled, pour half into the egg mixture then incorporate. Once incorporated, add the remaining chocolate mixture.
Once the egg and chocolate is all incorporated add the plain flour, peppermint extract and cacao powder and folder these in until all incorporated.
Add in the chocolate chips and the Mint Aero chocolate and mint matchsticks and pour into the prepared tin.
Pour half the mixture into the tin and spread it out. Then add some whole bits of mint chocolate then pour the rest of the mixture on top.
Add some whole chunks of mint chocolate to the top. Bake in the oven for 30 minutes or until flaky on the top but still soft in the centre.
Once, the brownies are cooked remove from the oven and leave to cool in the tin for five minutes then transfer to a cooling rack to cool fully.