Pre heat oven... 160c, fan

Cut kitchen foil into a square and line a square cake tin. Press foil carefully down even sides, and put to oneside. Also break up oero biscuits! And put to one side.

Melt butter in a pan, low to medium heat.

Crack eggs into a large bowl, add sugar and vanilla extract, mix well.

Pour in melted butter whilst it's very warm. Stir for 1 minute

Sieve the cocoa powder and flour, into the egg mixture, stir for 1 minute, getting right down to the bottom of bowl. So there are no streaks in flour. Lastly add broken Oreo pieces and fold in slightly, not too much

Spoon mixture into prepared tin. And bake on middle shelf, for 35 minutes, checking after 30 minutes carefully

Let cool fully. And turn out onto a chopping board and then cut into small squares