Chocolate Mashed Potato Cake with Tahini Drizzle | Veggie Desserts Potato in cake? Yup. Potato has been used in cake for hundreds of years. It adds bulk and moisture, plus helps the cake to keep for longer. You can’t taste the potato, but it adds a nice extra richness to this decadent chocolate cake. I love tahini, so I’ve added an extra dash of flavour by drizzling it over the cake.