Chocolate cake layers:

1. Preheat the oven to 350°F (180°C). Grease and flour three 9- or 10-inch round cake pans.
2. In a large mixing bowl, cream the butter and sugar together until light and fluffy. Beat in the eggs one at a time, then add the heavy cream, Greek yogurt, and coffee; mix until smooth. Add the salt, baking soda, and vanilla and mix to combine. Gradually stir in the cocoa powder and flour until just combined, then fold in the melted chocolate.
3. Divide the batter evenly between two of the prepared pans. Sprinkle the chocolate chips over the batter and swirl them in slightly with a knife.
4. Bake for 25–35 minutes, until a toothpick inserted in the center comes out clean. Cool completely in the pans, then run a knife around the edges, invert onto plates, wrap in plastic wrap, and refrigerate for at least 1 hour.

Nutella cheesecake layer:

1. Reduce the oven temperature to 325°F (160°C).
2. Beat the cream cheese until completely smooth with no lumps. With the mixer running, gradually add the sugar and beat until creamed. Add the eggs, salt, chocolate hazelnut spread, sour cream, and cocoa powder; stir until smooth and combined.
3. Pour the batter into the third prepared pan and bake for about 20 minutes, until the center barely jiggles when the pan is nudged. Cool completely, loosen the edges, and carefully invert onto a plate. Wrap in plastic and refrigerate for at least 1 hour.

Whipped Nutella frosting:

1. Pour the heavy cream into a chilled stand-mixer bowl fitted with the whisk attachment. Beat on high speed until stiff peaks form.
2. Add the chocolate hazelnut spread and softened butter and beat until combined and smooth.
3. Switch to the paddle attachment (or reduce speed). Gradually add the powdered sugar, vanilla, and salt, beating until the frosting is light and fluffy. Add more powdered sugar a few tablespoons at a time until it holds its shape and is thick enough to spread.

Assemble:

1. Trim the tops of all three layers so they are level.
2. Place one chocolate cake layer on a cake board or serving plate. Spread a thin, even layer of frosting over the top. Set the cheesecake layer on top and gently press to adhere. Spread another thin layer of frosting over the cheesecake layer. Place the second chocolate cake layer on top.
3. Frost the top and all the sides of the assembled cake with the remaining frosting, smoothing as you go. You may have some frosting left over.
4. If desired, dust the top with cocoa powder and press pirouette cookies upright around the sides. Pile chocolate curls and shavings on top.
5. Refrigerate until ready to serve. Slice with a warm, clean knife.