Method

1. Line the base and sides of a 8×8” loose base/ push up tin with grease proof paper / parchment paper.
2. In a food processor / blender, add in 500g of the Oreos and blitz until fine and crumb like.
3. Melt the butter either on the hob in a saucepan or in the microwave. Add the melted butter into the crumbled Oreos, it should be sticky and stick together when pressed together with your fingers.
4. Pour half of the mixture into the lined tin, compact this into the base of the tin and get right into the edges.
5. In a medium sized mixing bowl with a electric hand whisk or in a stand mixer bowl with balloon whisk attachment, add in the whipping cream and condensed milk. Whip this on high speed until thick and holds a peak, this’ll take around 3-5 minutes on high speed.
6. Blitz the remaining 100g of Oreos in a blender then fold into the ice-cream.
7. Pour the ice-cream over the layer of Oreo base. Using a off-set spatula or back of a spoon, spread the ice-cream out in an even layer making sure to get right into the corners of the tin. Tap the tin gently on the worktop (this will pop any air gaps / bubbles).
8. Add the remaining buttery Oreo crumbles on top in a even layer on top of the cream. Use your hands to press the mixture very gently into the cream so it sticks. Refer to video for example. 
9. Wrap the tin in some clingfilm / plastic wrap then place the tin in the freezer and allow to set firm. Anywhere from 6 hours to overnight (preferably overnight!).
10. Once the ice-cream feels firm / set, push the bars out the tin, or lift the ice-cream slab out of the tin by lifting it by the paper overhangs.
11. Use a sharp knife, cut them into bars. TIP: It helps to warm up a knife in some hot water, dry with some kitchen paper or tea towel then cut. 
12. Serve and enjoy.