Preheat oven to 325F. Also, prep a 9inch baking pan with butter. Also, I like to coat my baking pan with flour after using butter. Then place the pan in the fridge to cool.
In a bowl or mixer bowl, combine the all purpose flour, hazelnut flour, cocoa powder, sugar, salt.
Add butter into the dry ingredients and slowly begin to cream. You want to mix until a consistent crumb-like texture forms.
Add in the eggs with the vanilla and buttermilk. Slowly drizzle into the mixing bowl while mixing. Be careful not to the over-mix batter. Mix until the batter just barely comes together.
Take the prepared baking pan out of the fridge/freezer. Pour in the chocolate hazelnut cake batter. Place in oven
Bake at 325F for about 55 to 60 minute until an inserted toothpick comes out clean.
Allow the cake to cool down for about 15-20 minutes before removing from baking pan. Let it cool down another 45 minutes before cutting. If decorating, allow it to further cool down completely before frosting**.