Prepare the brownie mix per the instructions. You can either make two square pans or 1 large 9x13" dish.
After the brownies are cooked remove and allow to cool completely before layering the trifle.
Mix the chocolate pudding with the milk and allow to set in the refrigerator.
Once the brownies have cooled, add a layer (using your fingers) in the bottom of a trifle dish.
In a large mixing bowl add the 2 cups of heavy cream and beat using a hand mixer, until stiff peaks form.
Add ¾ Cup Nutella to the cream and beat again until well mixed. Set aside.
Using a spoon drizzle spoons of the remaining Nutella over the first layer of brownies on the bottom of the trifle dish.
Spoon half of the chocolate pudding on top.
Add a layer of hazelnut wafer cookies (reserving some to crumble on top).
Spoon half of the nutella cream over the top of the cookies.
Add another layer of brownies on top.
Spoon the remaining chocolate pudding on top of the brownies again.
Top with the remaining nutella cream and sprinkle crushed hazelnut cookies on top for decoration.
Allow to sit in the refrigerator to "set up" for a few hours before serving.
Notes
There's no right or wrong way to put a trifle together, it's very forgiving. You may have some ingredients left over (like brownies, I had about a good 6 brownies left).