2. Whisk together sugar, oil and eggs in a large mixing bowl. Whisk in potatoes. Stir together flour, cocoa, baking powder, cinnamon, baking soda, nutmeg and salt in another bowl. Alternately add the dry ingredients and the buttermilk to the egg mixture, beginning and ending with the dry ingredients and stirring with a spoon or rubber spatula.
3. Spoon the batter into the prepared pan. Bake until the top springs back when touched lightly, 30 to 35 minutes. Let cool on a rack for 10 minutes. Invert the cake onto a rack and let cool thoroughly. Transfer to a plate and sift confectioners’ sugar over top.