Preheat oven to 180 degree C. Line a pan with parchment paper and set aside.
Take butter and chocolate in a microwave safe and bowl and melt it for 20 to 30 seconds till melted.
Now remove it and mix well. Set aside.
Take eggs in a bowl, add in brown sugar and whisk till smooth.
Add in chocolate mix in and mix well.
Add in flour and cocoa powder and mix well.
Add in chopped mars chocolate and mix well.
Spoon this into the pan and bake for 20 to 25 mins.
Remove and cool completely,.
Slice and serve.