From the blog For Love of the Table
3 c. all-purpose flour
2 t. baking powder
2 t. baking soda
2 t. cinnamon
1/2 t. ginger
1/4 t. ground cloves
1/4 t. nutmeg
1 t. salt
1 c. walnuts or pecans—lightly toasted and coarsely chopped
2 c. mashed cooked sweet potatoes—about 18 oz. of purée (see note)
1 1/2 c. vegetable oil
1 1/2 c. sugar
1 t. vanilla
4 large eggs, at room temperature
4 oz. semi-sweet chocolate, melted and cooled
Butter and flour a 10-inch tube pan or 2 8-cup loaf pans. Set aside. Whisk the dry ingredients together. Stir in the nuts and set aside.
In a stand mixer, using the whisk attachment, beat the sweet potatoes, sugar, oil and vanilla at medium speed until very smooth and glossy. (Or, whisk by hand in a large bowl). Whisk in the eggs one at a time, making sure each egg is fully incorporated before adding the next. By hand, fold in the dry ingredients and the nuts. Scoop one third of the batter (about 21 oz.) into a separate bowl and quickly stir in the chocolate.
Place the batters in the pan(s), alternating as for a marble cake. Then, with the end of a wooden spoon or the blade of a table knife, gently draw swirls through the batter to marbleize it. Don't over mix or you won't have a marble affect—two, zig-zag passes through the pan should be sufficient.
Bake in a 350° oven until the cake springs back and a toothpick inserted in the center comes out clean—about an hour for the loaves and an hour and 10 minutes for the tube cake. Cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
Note: You will need about 2 lbs. of sweet potatoes to produce 2 cups of purée. Roast the sweet potatoes in a 400° oven. Prick the sweet potatoes in several spots with a fork or paring knife and transfer to a baking sheet. Bake until easily pierced with the tip of a knife, about 40 to 60 minutes—the potatoes will have begun to ooze a bit of sugary syrup when they are fully cooked. When cool enough to handle, cut open the sweet potatoes and scoop out the flesh. Purée in the food processor, or press the flesh through a sieve or mesh strainer. A sweet, orange fleshed variety of sweet potato (like Garnet, Jewel or Beauregard) will give the best result for this cake.
(Recipe adapted from The Victory Garden Cookbook, by Marian Morash)
http://www.forloveofthetable.com/2013/10/marbled-chocolate-sweet-potato-cake.html