Place biscuits into the TM bowl and crush on Speed 8 for 15 seconds or until they resemble fine crumbs (a few chunks is fine!). Set aside in a separate bowl.
Place the butter, hazelnut chocolate and condensed milk into the TM bowl and heat for 5 minutes, 80 degrees, Speed 2 (scrape down the sides every couple of minutes).
Add the biscuit crumbs and mix on Reverse, Speed 3 for 20 seconds or until completely combined.
Press down into a 20X30cm lined slice tin. Place in fridge.
Place the dark chocolate and coconut oil in the TM bowl. Melt on 80 degrees, Speed 2, 4 minutes (scraping down the sides partway through).
Pour over slice and swirl with a knife to get a lovely effect.Quickly place the Ferrero Rocher balls in straight lines on top of the melted chocolate.
Place in fridge for at least 3 hours (overnight is best).
Take the slice out of the fridge and leave for 10 minutes before cutting (this will help to prevent the chocolate from cracking).
Cut the slice into squares leaving one Ferrero Rocher ball on each slice.
Store in the fridge in an airtight container for up to a week.