Grease and line an 18cm x 28cm rectangular slice tin with baking paper and set aside.
Place the butter into the Thermomix bowl and melt for 2 minutes, 80 degrees, Speed 2 (or until melted).
Add the Oreo biscuits and crush for 10 seconds, Speed 8. Press the mixture firmly into the base of the prepared tin. Place into the fridge to chill.
Place the Nutella, cream cheese and icing sugar into a clean Thermomix bowl. Mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat a further 2-3 times or until smooth and creamy.
Spread the mixture over the prepared base.
Sprinkle the crushed Ferrero Rocher chocolates over the top and place into the fridge for a minimum of 4 hours (preferably overnight) to set - see notes.