Line an 8" baking tin with parchment paper and set it aside. Preheat the oven to 180C/350F while you make the brownies.
Melt the butter in a small bowl. I do this in the microwave in short burst of 10-20 seconds, swirling or stirring in between. Add in the cocoa powder, vanilla and whisk it together until silky smooth.
In a separate larger bowl, whisk together the eggs and sugar for about five minutes until really pale and at least double in size. Slowly drizzle in the chocolate mixture, beating as you go so it is all evenly mixed together.
Into the mixing bowl, tip in the flour and salt. Fold this into the chocolatey mixture rather than whisking so you keep the volume added to the batter. Finally, tip in the larger M&Ms and fold through.
Pour the brownie batter into the prepared baking tin and ease it into the corners levelling the top. Scatter the mini M&Ms over the top of the brownie mixture, covering the top evenly.
Bake in the preheated oven for 25-35 minutes the edges feel firm to touch and the middle has the tiniest wobble. It will take more or less time, depending on your oven. For me, my brownies are done around 27-28 minutes. Leave the brownie in the tin to cool completely before cutting into squares. Enjoy!