1. Preheat oven to 350 degrees. Butter and flour a 10-inch fluted or straight-sided cake pan.
2. Place butter, Nutella® and sugar in the bowl of an electric mixer. Cream together on medium speed until light and fluffy. Add egg yolks one at a time, mixing well after each. Add the buttermilk and mix again.
3. In a food processor fitted with the sharp blade, add the cooled nuts, flour, baking powder, baking soda and salt. Grind until the texture resembles very fine sand.
4. Add the dry mixture to the wet, and mix on slow speed until just incorporated. In a separate bowl, beat the egg whites to soft peaks and gently fold into the cake batter.
5. Pour batter into the prepared pan and spread out. Place in the oven to bake 45 to 50 minutes, or until a toothpick comes out clean. (This cake tends to fall a little in the center, so don’t be upset if this happens.) Cool and remove from the pan.