Preheat oven to 160 degrees celsius (fan-forced) and line a 20cm square tin with baking paper. Set aside.
Place the butter into a microwave-safe bowl. Heat for 1 minute. If using a Thermomix - Place the butter into the Thermomix bowl. Melt for 3 minutes on 80 degrees, Speed 2.
Add the caster sugar and sifted cocoa powder and stir. Heat in the microwave on 30 second bursts (50% power) until the sugar has almost dissolved and the ingredients have combined - stirring each time.If using a Thermomix - Add the caster sugar and cocoa powder and mix for 10 seconds on Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds.
Allow to cool slightly.
Add the vanilla extract and the eggs and whisk until combined.If using a Thermomix - Add the vanilla extract and the eggs and mix for 10 seconds on Reverse, Speed 4. Scrape down the sides and repeat.
Sift the plain flour into the bowl and gently fold through.If using a Thermomix - Add the plain flour and mix for 5 seconds on Speed 4. Scrape the sides of the bowl and repeat.
Pour half of the brownie mixture into the prepared tin. Place 12 Oreo cookies into the mixture.Pour over the remaining brownie mixture.Chop the remaining Oreos and press on top of the brownie mixture.
Bake for 30 minutes or until cooked through.Leave in the tin to cool completely.
Notes
RECIPE NOTES & TIPS:
Line your tin with baking paper - to make it easier to remove the brownies from the tin once cooked.
Don't over mix the brownie batter - this will cause the brownies to become cake-like rather than fudgy.
Don't overcook the brownies - otherwise they will become dry.
For fudgy/moist brownies - cook for 25-30 minutes and test with a toothpick which should come out with moist (but not wet) crumbs on it.
The brownies will continue to cookas they cool.
Cool the brownies before cutting - this will make it easier to slice them neatly.
Storage - store brownies in an airtight container at room temperature for up to 4 days, or in the fridge for up to 5 days.
Freezing - freeze in an airtight container in the freezer for up to 3 months. To thaw, leave the slices in a container in the fridge overnight, or sit them at room temperature for 1-2 hours.