First Layer (Nutella Mousse): Whip cream until you almost have stiff peaks. Add the Nutella into the cream gradually (2 to 3 smaller portions), continuing to whip gently until combined. Spoon the Nutella Mousse into the bottom of serving glasses, about 1/3 of the way up the glass. Place in the refrigerator until it has set, about 30 minutes.
While the Nutella Mousse layer is setting, prepare the next layer.
Second Layer (Dark Chocolate Mousse): Melt dark chocolate chips with the unsalted butter over a double boiler. When the mixture is halfway melted, add two tablespoons of hot water to the mixture. Stir occasionally until the mixture is smooth. Remove chocolate mixture from heat and cool slightly. Whip cream until soft peaks form. Separate eggs, then, whip the egg whites until they begin to form soft peaks. Add in the sugar. When the chocolate mixture has cooled slightly, stir in the yolks until combined. Fold in whipped cream and the whipped egg whites, adding thirds of each mixture in an alternating fashion, and mix until well combined. Spoon the dark chocolate mousse on top of the Nutella layer, about 2/3 way up the glass. Place in the refrigerator until it has set, about 30 minutes.
While the Dark Chocolate Mousse layer is setting, prepare the next layer.
Third Layer (Milk Chocolate Mousse): Melt milk chocolate chips with the unsalted butter over a double boiler. When the mixture is halfway melted, add two tablespoons of hot water to the mixture. Stir occasionally until the mixture is smooth. Remove chocolate mixture from heat and cool slightly. Whip cream until soft peaks form. Separate eggs, then, whip the egg whites until they begin to form soft peaks. Add in the sugar. When the chocolate mixture has cooled slightly, stir in the yolks until combined. Fold in whipped cream and the whipped egg whites, adding thirds of each mixture in an alternating fashion, and mix until well combined. Spoon the milk chocolate mousse on top of the dark chocolate mousse layer, almost to the top of the glass. Place in the refrigerator until it has set, about 30 minutes.
Serve cold and top with whipped cream if desired.