Preheat the oven to 180ºC.
Grease and line a 20cm square cake tin with oil and baking paper.
Sieve the flour and cocoa into a bowl.
Place the butter and plain chocolate into a bowl of water over a pan of hot (barely simmering) water to melt, stirring with a wooden spoon.
Remove the bowl from the pan and allow the mixture to cool for a few minutes.
Stir in the beaten eggs, Muscovado sugar, flour, cocoa and chocolate chips mixing well.
Spoon half the mixture into the tin, spreading it out evenly.
Press 8 miniature Oreos in rows of 4 by 4 into the mixture.
Pour over the remaining mixture and spread out evenly again.
Decorate the top with the remaining 8 miniature Oreos in rows of 4 x 4.
Place in the centre or the oven and bake for 27 minutes or until the top forms a crust and the centre is slightly springy and doesn’t wobble when shaken.