Preheat oven to 160 degrees celsius (fan-forced). Grease and line a 18cm x 26cm rectangular slice tray with baking paper and set aside.
Place the butter into the Thermomix bowl and melt for 2-3 minutes, 80 degrees, Speed 2, (or until melted).
Add the brown sugar and cocoa powder and mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Add the vanilla extract and eggs and mix for 10 seconds, Reverse, Speed 4. Scrape down the sides and repeat.
Add the self-raising flour and mix for 5 seconds, Speed 4. Scrape the sides of the bowl and repeat.
Spoon the mixture into the prepared tin (it will be thick) and spread out evenly. Sprinkle the M&Ms over the top.
Bake for 20-25 minutes or until the top has hardened with a crust and the centre is soft (but not gooey).
Allow to cool completely in the tin before cutting into slices. Serve warm or cold.
Store in an airtight container for up to 5 days.