Preheat your oven to 180ºc/160ºc fan, and line a 9x9" square tin with parchment paper.
Melt the dark chocolate with the butter and leave to cool slightly
Whisk together the eggs and sugar until really thick and doubled in volume
Fold the chocolate/butter mix into the eggs/sugar mix carefully
Add in the plain flour and cocoa powder, and fold that in as well
Add the M&Ms and fold through
Pour the brownie mixture into the lined tin
Bake the brownies in the oven for 25-30 minutes, or until baked through
Leave the brownies to cool fully in the tin, and then set in the fridge for 3+ hours if you can for the best brownie
Decorate with a drizzle of melted chocolate and crushed M&Ms if you fancy
Notes
I use regular M&Ms, but you can use any. If you use filled ones, you may want to substitute 100g of the M&Ms for chocolate chips so there is enough gooeyness
These will last for 4-5+ days at room temp or in the fridge