Cook the sweet potato. Preheat oven to 400°F. Line baking sheet with foil or parchment paper. Use a fork to poke several holes into sweet potato and set on baking sheet. Roast 40-50 minutes, or until easily pierced with a fork. Allow potato to cool significantly. Scrape the flesh out of the skin and use a fork to mash in a bowl. Measure out 1 cup of puree. Reduce oven temperature to 350°F. Prepare a 12-cup muffin pan with nonstick spray or muffin liners.
In a large bowl, gently whisk flour, pumpkin spice, baking powder, baking soda, and salt.
In a separate medium bowl, whisk together mashed sweet potato, brown sugar, buttermilk, oil, eggs, and vanilla. Add liquid ingredients to dry ingredients. Using a spatula, fold ingredients just until combined. Do not overmix.
Use a ¼ cup portion scoop to divide out the batter. Drop ½ Tablespoon nut butter in center of each muffin. Push to the center of muffin by bobbing your spoon up and down. Cook for 18-20 minutes, or until a toothpick inserted into muffin (not nut butter) comes out clean. Enjoy!