Place a stand mixer bowl and whisk attachment in the freezer.
Fill a piping bag with 45 grams of Nutella. Snip a tiny bit off the top.
Set the mold with the 3x1.5cm holes on a small baking tray. Pipe Nutella into 6 of the molds.
Tap the tray a few times against the counter to even out Nutella. Place in the freezer.
Sprinkle gelatin over cold water in a microwave-safe bowl or mug. Mix.
In a small saucepot, add in dark chocolate (chopped into chunks or chips), Nutella, and heavy whipping cream into a saucepot. Melt over medium heat and whisk until until cobined, and shiny. Remove from the saucepot from heat.
Melt the gelatin in the microwave for 15 seconds and whisk into the chocolate Nutella mixture until completely incorporated. Set aside for 10 minutes.
Remove stand mixer bowl from the freezer, using a stand mixer, add in the rest of heavy whipping cream and using the whisk attachment whisk until stiff peaks form.
Add ⅓ of chocolate Nutella mixture to create and fold until combined. Repeat 2 more times with the remaining chocolate Nutella mixture.
Using a silicone mold on a small baking tray, fill up all molds halfway with Nutella mousse. Tap it a few times against the counter to remove any air bubbles and even out the top surface.
Remove the small Nutella semi-spheres from the freezer and add one to each of the mousses.
Fill with the rest of the mousse and tap it again against the counter to remove any air bubbles. If necessary, use an offset spatula to spread the mousse as smooth and even out the surface. Place into the freezer and let freeze until completely firm, at least 6 hours or overnight if possible. *****