Preheat oven to 350 degrees. Spray three 6-inch cake pans with non-stick spray and line the bottom with parchment paper; set aside.
CHOCOLATE CAKE: Pour the cake mix into a large bowl. Make a “well” in the center and add eggs, butter and milk. Beat the mixture for 2 minutes, scraping the sides of the bowl if needed. Pour the batter evenly into the prepared pans. Bake for 20-25 minutes or until a toothpick inserted into the center of the cake comes out clean.
Remove pans from the oven to a wire rack. Cool in pans for 5 minutes. Run a knife around the edges of the cake pans to loosen the cake. Invert the cakes and place back on the wire rack to cool completely. Use a long serrated knife, to cut the tops off the cakes to level them.
NUTELLA BUTTERCREAM FROSTING: In a large bowl, using an electric hand mixer beat Nutella and butter until well combined. Add confectioners’ sugar a cup at a time alternating with 1 T. milk. Beat for 2 minutes until light and fluffy.
Spread a dab of frosting on a cake plate to hold the cake in place. Center one cake (cut side down) over the dab of frosting. (At this point I like to put pieces of parchment paper around the cake to keep the plate clean while I decorate) Frost the top of the cake. I pipe mine on, starting in the center and working my way to the edge or you could spread it with an offset spatula. Repeat with the two remaining layers; when you get to the top of the cake smooth with an offset spatula. Spread a thin coat of frosting on the sides of the cake, scraping with a spatula so you can see the naked edges. Refrigerate the cake while you prepare the ganache.
CHOCOLATE GANACHE: In a saucepan, over medium heat bring the heavy cream to a boil stirring frequently. Remove from heat and stir/whisk in chocolate until completely melted about 2 minutes. Allow chocolate to cool and thicken a bit, stirring/whisking occasionally. Slowly drizzle ganache in the center of the cake allowing it to drip down the sides.
Chill the cake in the refrigerator for a few hours before slicing and serving. Garnish with fresh fruit or flowers just before serving.