Preheat oven to 350ºF (180°C).
Butter and flour 9×13 inch (20x26cm) cake pan and line it with parchment paper, see Notes belwo.
Put the shredding disc in the food processor and shred 4 ounces russet potato. If you don’t have a large food processor, you may need to make the cake batter in two batches.
Change to the steel knife. If you had to remove the shredded potato momentarily put it back in the bowl of the processor.
Add 2 cups sour cream, 1 ¾ cups all-purpose flour, 1 ¾ cups sugar, ¾ cup unsweetened cocoa powder, ½ cup unsalted butter, 2 eggs, 1 ½ teaspoons baking soda, 1 ½ teaspoons baking soda, 1 teaspoon vanilla extract and ½ teaspoon salt and process for 3-4 minutes, stopping to scrape sides of the bowl and making sure nothing sticks to the bottom.
If you have to make it in batches transfer each mixture to a large bowl and then mix it all together.
Pour chocolate mixture into prepared pan.
Bake for 35 to 40 minutes, or until a cake tester or toothpick inserted in the center comes out clean.
Cool completely in the pan on a wire rack and remove onto a platter, running a smooth-bladed knife around the edges to make sure there are no stuck pieces and helping you with the parchment paper.
Frost with peanut butter chocolate frosting or dust with powdered sugar.