Lightly grease a 6” spring form pan with butter/oil and dust it with flour, the way you prepare a pan for baking.
For the bottom layer
Crush the Oreos and hazelnuts in a food processor / mixer until sand like texture.
Take it out in a bowl.
Drizzle melted butter over it and coat it until butter is well mixed.
Put the mixture in the greased pan pressing it down firmly with a back of a spoon or base of a small bowl to level the mixture evenly .
Place the pan in the fridge for 20 minutes to chill and set.
For the middle layer
Mix the 75 ml cream and the chocolate in a microwave safe bowl and melt it in the microwave on 50 % power until smooth and without lumps. It takes around a minute . Stop and stir every 15 seconds.
Alternately you can melt the chocolate with cream by using double boiler method.
Once chocolate is melted and shinny keep aside to cool down a bit.
Now in another microwave safe bowl melt the Nutella for 30 seconds to soften it up a bit . This will help loosen it and mix well later.
Keep it aside to cool a bit.
Now in a big bowl whip the 1.5 cups chilled cream until it starts to thicken. Now add in sifted corn flour .
Continue whipping until a bit stiffer peaks. Be careful, do not overmix or it will turn into butter and become useless.
By this time chocolate should be cool to touch .
Now add a big spoonful of stiffened chilled cream to the chocolate bowl and fold in gently with a spoon until mixed well.
This is done so that the chocolate doesn’t get a shock when mixed with the rest of the cream.
Now add the cream mixed chocolate back to the chilled cream bowl.
Give the Nutella bowl a vigorous 10 seconds mix with a spoon to loosen . Now add the Nutella to the chilled stiffened cream too .
Gently fold the chocolate and Nutella in by a spoon / spatula using cut and fold method . We want the mousse to be fluffy and airy.
Now take out the spring form pan from the fridge and gently spoon the mousse in it.
You may lick the leftovers .
Gently tap the pan on the kitchen counter to get rid of any air bubbles.
Level the mousse with the back of the spoon or an offset spatula.
Cover the pan with a cling wrap or a foil on the top .
Place it back in the fridge to chill for atleast 6 hours or overninght .
For top layer.
For chocolate ganache, place the chocolate in a bowl.
In another microwave safe bowl heat the cream until a simmer , not boiling . You may heat up the cream in a saucepan on stovetop too .
Now pour the hot cream over the chocolate.
Let it sit undisturbed for 10 seconds.
Now slowly start stirring it until chocolate is melted and shinny.
If chocolate is not completely melted you may re warm the mixture for few seconds.
Let it be cool to touch.
Now pour it over the set mousse and spread evenly.
Place it back in the fridge for 30 minutes to an hour and your ganache will be set.
Now remove the pan from the fridge .
With a sharp knife just loosen the edges of the pan in a circular motion, remember not to cut through the cake.
Gently open up the springform pan by pulling the tab and unlocking it.
Give the ring a wiggle and try to pull it up gently.
You may garnish and serve right away or refrigerate covered for upto 5 days.
For clean cake slices use a sharp knife to cut through the top layer, the mousse and the oreo layer.
Insert a spatula under the oreo layer to pull it out.