Preheat oven to 375 degrees F (350 if you pans are dark). Grease and flour two 9-inch cake pans.
In a large mixing bowl, combine flour, starch, baking powder, soda, and salt. Set aside.
In a medium mixing bowl, cream butter and sugar together. Beat in the eggs until smooth and then add the bananas and buttermilk, mixing until fully incorporated. Stir in the flour until just combined. Fold in the hazelnuts. Spread equally into two pans. Bake for 22-25 minutes until toothpick comes out clean. Let the cakes cool in the pans for 5 minutes on a cooling rack and then remove from pans onto rack to cool completely.
For the Frosting:
In a small sauce pan, whisk together cream and flour. Add milk and heat over medium-high heat whisking constantly. Cook until it turns into a thick sauce like pudding. Remove from heat and pour into a shallow container. Place in fridge or freezer and stir every 5 minutes until it has cooled to at least room temperature.
In the bowl of a standing mixer, use the whisk attachment to beat the butter and sugar for 2 minutes or until fluffy. Add the cooled milk mixture, Nutella, and vanilla and beat for 6-10 minutes until it comes together and is fluffy. If the frosting is too airy, switch to using the paddle attachment and beat on low for 2-4 minutes.
Place one cake on a cake stand and top with one fourth of the frosting. Place the second cake on top upside-down. Apply a thin layer of frosting on the top and sides of cake to create a crumb layer. Place cake in the fridge for 30-60 minutes and then frost the top and sides. Pipe additional decorations on top. Sprinkle the top of the cake with chopped hazelnuts and press more into the sides.