Preheat oven to 375°F (190°C, or gas mark 5). Pierce the sweet potato all over with a fork and bake directly on the middle rack of the oven for about 1 hour, or until tender all the way through. Alternatively, cook the sweet potato in a microwave on high, turning over once, about 10 minutes, or until tender. Let the sweet potato cool slightly, then peel and purée either with a potato ricer or masher. You’ll need 1 cup (255 g) of purée.

Butter a 9 x 9 x 2-inch (23 x 23 x 5 cm), or similar size, baking pan. In a large bowl, whisk together the 1 cup (255 g) puréed sweet potatoes, eggs, buttermilk, yogurt, and lemon zest. Place the cornmeal, flour, baking powder, baking soda, salt, sugar, ginger, and cayenne pepper in a food processor, and pulse until combined. Add the butter to the food processor, and pulse until the mixture resembles coarse meal.

Add this cornmeal mixture to the sweet potato mixture, stir until just combined, and pour into the prepared baking pan. Bake 35 to 45 minutes, or until the corn bread is golden brown on top and a paring knife inserted into center comes out clean. Let cool slightly before serving.