In a large mixing bowl, whisk the pumpkin, oil, eggs, sour cream, sugars and spices. Make sure everything is well combined.
In a separate small bowl whisk together the flour, soda and salt. Fold the dry ingredients into the wet, mixing just until combined. Fold in the walnuts.
Spread the batter evenly in a lightly greased 9 inch spring-form pan.
Bake for about 50 minutes, until a toothpick inserted near the center comes out without wet batter clinging to it.
Cool briefly before removing from the pan.
Make the frosting by mixing together the Nutella, sour cream and powdered sugar. Add just enough milk or cream for a spreadable consistency. Add a little more sugar if the frosting seems too thin. Spread on the completely cooled cake.
Notes
For this and all baking recipes, make sure to measure your flour using the Fluff - Scoop- and Fold method: first fluff up the flour to loosen it, then lightly scoop it into your measuring cup, and then level off the top. This will help insure that you get an accurate amount of flour.