2 cups chocolate hazelnut spread, room temperature
1/4 cup hazelnuts, coarsely chopped
Break the cookies into the bowl of a food processor. Add the butter and process until the mixture starts to clump. Tip into a 23 centimeter springform pan, and press into the base with your hands, or the back of a spoon. Chill in the fridge for 30 minutes.
In a large bowl, beat the mascarpone and powdered sugar until smooth. Add the chocolate-hazelnut spread and continue beating until combined and smooth.
Carefully smooth chocolate-hazelnut mixture over the cookie base. Cover with chopped hazelnuts, and chill in the fridge for at least two hours (or overnight). Using a small metal spatula, loosen the sides of the cake from the pan, and spring it out of the pan. Serve straight out of the fridge for best results.