Pre heat oven to 160c... use middle shelf.
Prepare tin with foil, dull side up.... make sure bottom and sides of tin are lined.
I use any square brownie/cake tin

Melt butter on medium heat

Crack eggs into a heat proof bowl, tip sugar in and mix with a hand mixer, add vanilla extract. Mix for 1 minute.

Pour in hot melted butter.
Stir for around half a minute

Add cocoa powder, and flour into the mix, stir for minute until all combined.

Spoon quarter of mix into prepared tin, and lay out 12 whole oreos spaced out.
Press them down lightly.
Pour carefully the rest on top and spread very carefully so even.

Then crumble 4 oreos all over on top.
Pop on middle shelf for 30-35 min, on 160c.
Fan oven.

Leave to cool, before cutting.