Preheat oven to 200C / 400F. Rinse and dry the sweet potato and place on a baking sheet. Roast the sweet potato until cooked, about 1 hour. Peel and mash the sweet potato. Measure 1/2 cup of sweet potato purée and set aside. (Eat the remaining sweet potato or mix it into regular mashed potatoes!)
Lower oven temperature to 175C / 350F. Line a muffin tin with paper liners. Set aside.
In a small bowl, whisk together flour, baking soda, baking powder, cinnamon, nutmeg and salt. Set aside.
In a mixing bowl, cream butter and sugar on medium-high speed until mixture is pale, light and fluffy, about 5 minutes, scrapping the sides occasionally. Add in egg and vanilla and beat on medium speed for about a minute until incorporated.
On medium-low speed, beat in the sour cream, followed by the reserved 1/2 cup of sweet potato purée. Using a spatula, stir in the dry ingredients until just incorporated.
Divide the batter among the paper liners and bake for 15 to 18 minutes until a skewer inserted into the center of the cake comes out clean. Let cool completely on a wire rack.