Grease and line a 20 x 20cm (8 x 8") baking tin (see my notes) and set aside.
Place condensed milk (250g) and butter (100g - cubed) in a large bowl. Place bowl in microwave and melt in 30 second bursts until butter is melted.
Add Nutella (110g) and mix to combine.
Add crushed sweet biscuit crumbs (250g) to Nutella mixture and mix until combined.
Press mixture into tin and smooth the top.
For the Nutella Topping
Place dark chocolate melts (225g) in a small bowl and microwave in 30 second bursts (mixing in between) until melted.
Add extra Nutella (3 tbsp) into melted chocolate and mix to combine.
Pour chocolate and Nutella mixture evenly over base and refrigerate until the slice is set.
I have used Arnott's Marie biscuits for this recipe, but you could substitute another plain sweet biscuit/cookie if you like.
Don't have a tin that's the right size? Feel free to use any suitable size container you may have on hand. I'm thinking of my overflowing Tupperware cupboard! Because this slice isn't baked in the oven you aren't restricted to a "baking" tin as such.
Even though these Nutella bars are no-bake, we still need to grease and line the baking tin/container with parchment paper, leaving some overhanging on each side. This gives you something to grip onto when removing the slice from the tin. If you are still having trouble getting it out of the tin, sit the base of the tin/container in hot water for a few seconds then give it another tug and it should come out.
*Please note that the amount of calories per serve is provided as a guide only, as ingredients and cooking methods can vary greatly*