This is one of my favorite vegetable cakes. It is tender, faintly spicy, and has a light orange crumb. Plus, it’s large enough to feed a crowd—perfect for holiday parties or whenever a festive “show off” cake is in order.–The Old Farmer’s Almanac Everyday Baking Cookbook
For sweet potatoes: Preheat the oven to 400°F. Pierce each potato twice with a paring knife and place the potatoes on a foil-lined baking sheet. Bake for 45 to 55 minutes, until the centers are soft. (Check with a paring knife or fork.) 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 any unused potato flesh for another use.)
For 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- to 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 (preferably a large stand model) 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, then 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 long tester (such as 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.
Sweet cakes and breads sometimes develop a dark top crust. This browning is rarely more than a fraction of an inch deep, so it is unlikely to affect the flavor. If the color bothers you, dust the top with confectioners’ sugar.