Potato in a cake that isn't a latke? You bet! Mashed potato gives this dessert body and a super moist texture. Martha made this on episode 704 of Martha Bakes.
Preheat oven to 350 degrees. Brush a 12-cup Bundt or tube pan with butter. Dust with cocoa, tapping out excess.
Peel potato and coarsely chop. Place in a small saucepan and add enough water to cover. Heat over medium-high, bring to a boil, and cook until tender. Drain, then mash until no lumps remain. Measure out 3/4 cup for the recipe; set aside. Reserve the remainder for another use.
Whisk together flour, salt, baking powder, and baking soda. In the bowl of an electric mixer fitted with the paddle attachment, combine butter and sugar and beat on medium speed until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating well after each addition. Add vanilla and melted chocolate and beat until thoroughly incorporated. Add potato and beat to combine. Reduce speed to low. Add flour mixture in 2 batches, alternating with sour cream, beating until just combined.
Transfer batter to prepared pan and smooth top with an offset spatula. Bake until a cake tester comes out clean, about 50 minutes. Transfer to a wire rack to cool for 20 minutes. Turn out onto rack to cool completely. (Cake can be wrapped in plastic and stored at room temperature up to 1 day or frozen up to 1 month.)
When cake has completely cooled, slice with a serrated knife, and serve with a dollop of whipped cream, if desired.