Nutrition per serving may change if servings are adjusted.
1 cup cake flour (see Tip), plus more for the pan
1 medium russet potato (about 12 ounces), peeled and cut into 1-inch pieces
1 cup whole-wheat pastry flour
1 teaspoon baking powder
¾ teaspoon salt
½ teaspoon freshly grated nutmeg
¾ cup granulated sugar
4 tablespoons ( ½ stick) unsalted butter, melted
¼ cup grapeseed oil
4 large eggs
2 tablespoons spiced dark rum
2½ tablespoons brown sugar
½ teaspoon ground cardamom
¼ teaspoon ground cinnamon
⅛ teaspoon ground pepper
Preheat oven to 350°F. Coat a 9-by-5-inch loaf pan with cooking spray, then dust with cake flour, tapping out any extra.
Place potato in a medium saucepan, cover with 2 inches of cold water and bring to a boil. Reduce heat to medium, cover and cook until tender, 12 to 15 minutes. Drain and return the potato to the pot to dry for 1 minute. Mash with a potato masher until mostly smooth. Measure out 1 packed cup (reserve the remaining potato for another use). Let cool for 5 minutes.
Sift cake flour, whole-wheat flour, baking powder, salt and nutmeg into a large bowl.
Pulse the mashed potato and granulated sugar in a food processor just until smooth. Add butter, oil, eggs and rum; process until smooth. Add the flour mixture and pulse until thoroughly mixed.
Combine brown sugar, cardamom, cinnamon and pepper in a small bowl. Scrape half the batter into the prepared pan. Sprinkle with the sugar mixture. Top with the remaining batter. Tap the pan on the counter to release any air bubbles.
Bake for 20 minutes. Reduce oven temperature to 325°F. Bake until golden brown and a toothpick inserted in the center comes out clean, 40 to 50 minutes more.
Run a knife around the edges of the cake. Let cool in the pan on a wire rack for 10 minutes. Turn the cake out onto the rack to cool completely, about 1½ hours.
Tip: Cake flour is finely milled and has a lower protein content than all-purpose or whole-wheat. The lower the protein, the less gluten the flour will produce, giving recipes a more tender texture.
285 calories;10 g fat(3 g sat); 3 g fiber; 42 g carbohydrates; 5 g protein; 44 mcg folate; 72 mg cholesterol; 16 g sugars; 15 g added sugars; 209 IU vitamin A; 2 mg vitamin C; 46 mg calcium; 2 mg iron; 213 mg sodium; 215 mg potassium