Make the crust: Add the crispy rice cereal to a zip-top bag, seal, and crush into crumbs with a rolling pin. Transfer the crumbs to a large bowl.
In a medium bowl, combine the chocolate chips and butter. Microwave in 30-second intervals, stirring between, until melted and smooth, about 1 minute total.
Pour the melted chocolate mixture over the cereal crumbs and stir to combine.
Grease 9½-inch (24-centimeter) pie dish with nonstick spray. Pour the crust mixture into the dish and press into an even layer against the bottom and up the sides of the pan.
Chill in the refrigerator while you make the mousse.
Make the chocolate hazelnut mousse: Microwave the chocolate hazelnut spread for 30 seconds. Set aside.
In a large bowl, beat the cold heavy cream with an electric hand mixer until soft peaks form. Add the sugar and beat to incorporate.
Using a rubber spatula, fold the warm chocolate hazelnut spread into the whipped cream until incorporated.
Remove the crust from the refrigerator and pour in the mousse. Spread in an even layer.
Cover the pie with plastic wrap and freeze for at least 6 hours, or overnight.
Make the crispy rice topping: Line a baking sheet with parchment paper.
Add the sugar to a medium nonstick skillet over medium-high heat. Cook without stirring for 1-2 minutes, until the sugar begins to caramelize. Swirl it around in the pan to help caramelize evenly.
Once the sugar is light amber in color, reduce the heat to low and add the butter. Use a rubber spatula to stir until the butter is melted, then turn off the heat. Add the crispy rice cereal and toasted hazelnuts and quickly stir to coat.
Transfer the topping to the prepared baking sheet and sprinkle with the salt. Use two forks to separate the mixture into smaller chunks, being careful not to touch the hot sugar with your hands. Let cool completely, about 15 minutes. The topping can be made 1 day ahead. Store in an airtight container once cooled.
When ready to serve, remove the pie from the freezer. Decorate the top with the crispy rice topping. Let the pie sit at room temperature for 10 minutes before slicing with a sharp knife. Warm the blade in hot water and wipe between each cut for clean slices.