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 and cacao powder and folder these in until all incorporated.
Add in the chocolate chips and about 150g of the M&M's, fold these into the mixture. Once all folded in, pour into the tin and spread out.
Bake in the oven for 30 minutes or until flaky on the top but still soft in the centre.