In a food processor, pulse the graham crackers and hazelnuts until they resemble cornmeal. Add the butter and pulse a couple more times until the crumbs are evenly coated. Divide the graham cracker mixture evenly among four small trifle dishes or other dessert containers. Place the dishes in the refrigerator.
Using the whisk attachment, add the cream to the bowl of a stand mixer and whisk on high speed until thick whipped cream forms. Transfer the whipped cream to a bowl and set aside.
Change the attachment to a beater and add the cream cheese (as much or as little as you like) and Nutella to the bowl. Mix on high until thoroughly incorporated. Then, add half of the whipped cream to the Nutella mixture.
Use a spatula to fold in the whipped cream. Add the remaining whipped cream, reserving a small amount for topping, and fold until evenly mixed. Refrigerate the reserved whipped cream.
Evenly fill the dessert trifles with the mousse and refrigerate for a minimum of four hours. Meanwhile, chop the extra hazelnuts for the garnish and toast them on the stove until golden brown. Set aside to cool completely.
In a microwave-safe bowl, add a few red melting chocolates and heat in 30-second intervals in the microwave. Stir between intervals until the chocolate is completely melted.
Fit a plastic piping bag with a #1 decorating tip. Fill the bag with the melted chocolate and begin piping red hearts onto a wax-lined baking sheet. Place the hearts in the freezer to set.
Remove the mousse from the refrigerator when ready to serve. Top with whipped cream, toasted hazelnuts, and a chocolate heart.
Notes
If you do not have a #1 decorating tip, you can simply snip the end of the piping bag with a pair of scissors. Make sure the cut is very small, so the heart outlines are more refined.