This is one of my favorite fall/winter desserts or breakfast treat. It's tender, moist, spicy and perfect for any large gathering when you want to 'show off,' a little bit. The extra step of baking & processing the potatoes will be worth the effort.
For sweet potatoes: Preheat the oven to 400°F. Pierce each potato a few times with a fork and place on a foil-lined baking sheet. Bake for 45 to 55 minutes, until the centers are soft. (Check with a wooden skewer.) Carefully slice the potatoes in half to let off steam. When they are cool enough to handle, scoop out 2 cups of flesh and place it in a food processor. Add the buttermilk and process until smooth. Refrigerate to cool. (Refrigerate and store any unused potato flesh for another use.)
For the cake: Adjust your oven rack to one setting below the center position, with no racks above it. Set the oven to 350° F. Butter a 10-inch (12 -14 cup) bundt pan lavishly. Sprinkle the nuts evenly around the bottom of the pan, then sprinkle with 2 tablespoons of the brown sugar. Set aside.
Sift the flour, baking powder, baking soda, salt, cinnamon, and nutmeg into a large bowl. Set aside.
Using an electric mixer on medium, cream the butter, then gradually beat in the sugar and 1 cup of brown sugar. Add the eggs, one at a time, beating well after each addition. Add the vanilla and orange zest and beat to blend. Add the sweet potato mixture to the butter mixture; beat on low until evenly blended. Add the flour mixture, about a third at a time, mixing on low after each addition. Turn the batter into the prepared pan, spreading it evenly.
Bake for 60 to 70 minutes, until a thin wooden skewer inserted into the center comes out clean. Cool in the pan for 15 minutes. Invert the cake onto a large platter. Cool for at least 30 minutes more before slicing.