Line your cake ring with cake collar and plastic film over the bottom, to prevent mousse from leaking.
Prepare bottom layer. In a small bowl add the gelatin powder and the cold water and let swell for 5-10 min. Add the hazelnut nougat and the heavy cream to a small saucepan and place on low heat, stirring until the nougat has melted. Remove from heat. Melt the bloomed gelatin over a baine marie, then add to the nougat mixture. Let cool down while whipping the cream with a hand mixer, until soft peaks form. Gently fold the nougat mixture into the whipped cream with a spatula. Pour in your lined cake ring then place in the freezer for at least 40min.
Prepare the middle layer, using the same process as in the previous step. Pour over the bottom layer that has now set and return to freezer for 40 more minutes.
Prepare the top layer. Let the gelatin bloom. Melt the chocolate with the heavy cream, remove from heat, then add the orange liqueur and stir well. Melt the bloomed gelatin and add to the chocolate liqueur mix. Let cool. Meanwhile whip the chilled cream together with the powdered sugar, until soft peaks form. Gently fold the chocolate liqueur mixture into the whipped cream with a spatula. Pour the top layer in your cake ring and return to the freezer. Let set for at least 4h.
After the cake has set in the freezer, remove the plastic film from the bottom and place on your serving plate. Prepare the ganache. Add the chocolate and heavy cream to a heat resistant bowl, and melt over a baine marie. Let it cool down a bit, but make sure it is still fluid. Pour gently on top of the mousse layer cake. Shake a bit or use an offset spatula to nicely level it. Let set in the fridge for a couple of hours.
After the ganache has set, it is time to decorate. Prepare your piping cone out of parchment paper (check link above on how to do it). Melt the white chocolate over a baine marie. Pour into the cone and decorate to your liking. Add some candied orange peel as well. Keep in the fridge until serving. Enjoy!