Preheat oven to 345 degrees and line pans with parchment paper on the bottom and grease the sides.
Sift cake mixes into a bowl to remove lumps and set aside.
In a large bowl, combine bananas, eggs, oil, buttermilk, sour cream, vanilla extract and cinnamon until smooth.
Stir in cake mix.
Divide evenly into 3 round pans.
Bake for about 27-35 minutes or until an inserted knife comes out clean. (I baked two of the pans and then the other one separately.)
Let cool completely.
To make buttercream beat butter and hazelnut spread. Add cocoa powder, powdered sugar, vanilla extract and add 1 Tablespoon of milk at a time until you reach your desired consistency. You will want a looser consistency than you would have for cupcakes.
To assemble, center your first layer of baked cake on your cake plate, then frost the top of that cake...then put another layer of cake...then another layer of frosting...then cake...then frost the entire cake! Use chopped hazelnuts to decorate.