• Preheat the oven to 350°. Generously butter the springform pan. To make sure it's easy to remove the bottom, after baking cut a piece of parchment paper into a circle, and place it in the bottom of the pan.

Make the streusel/topping

• In a medium bowl, combine all ingredients, until uniformly mixed. Set aside.

Make the batter

• In a large bowl, or the bowl of a stand mixer, cream together the butter and sugars. Scraping down the sides if needed, beat until very fluffy and paler in color.
• Beat in the eggs also until very fluffy.
• Beat in the sour cream, sweet potato, and vanilla, until very well combined and creamy.
• Beat in the baking powder, baking soda, salt and spices.
• Beat in the flour, until well combined - scrape sides of bowl as needed.
• Place half of the batter in the prepared pan, leveling out with a spatula or rubber scraper. Sprinkle half of the topping mixture over the batter.
• Evenly spread the remaining batter over the topping layer, taking care not to mix the layers together.
• Evenly distribute the chopped nuts over the batter, and finally the remaining topping mixture.
• Bake for approximately an hour, checking for doneness at about 50 minutes. A toothpick inserted near the center should come out completely clean. It is better to leave it baking for a few extra minutes, rather than having it be underdone. It's not going to dry out. A 9 x 13 pan will be done in probably about 45 minutes.
• Set the cake on on cooling rack, removing the springform ring after about 5 minutes.
• When ready to serve, use an offset spatula, or a long, thin knife, to remove the pan bottom and place the cake on a plate. You can leave the springform bottom on the cake, but I don't like to get mine scratched up, so I take it off.
• Cut into wedges to serve. You can, as I did, mix some cream and 10x sugar together to make a drizzle to serve with it.