Preheat the oven to 425°F / 218°C. Line a baking sheet with parchment paper.
Wash and dry the sweet potatoes thoroughly.
Pierce each sweet potato 5 to 6 times with a fork.
Place the sweet potatoes on the prepared baking sheet.
Roast for 40 to 45 minutes, or until the sweet potatoes are fork tender.
Remove the sweet potatoes from the oven and allow them to cool to room temperature.
Scoop the sweet potato flesh into a food processor.
Blend until smooth.
Measure 1 1/4 cups of pureed sweet potato for the muffin batter. Set aside.
Alternatively, use 1 1/4 cups canned sweet potato puree.
In a mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt until well combined.
In a separate mixing bowl, whisk together the oil or melted butter, light brown sugar, granulated sugar, eggs, and sour cream until fully combined.
Add the pureed sweet potato to the wet ingredients.
Whisk until the mixture is smooth and evenly combined.
Add the dry ingredients to the wet ingredients.
Mix gently until just combined. Do not overmix.
Cover the bowl with a clean dish towel or lid.
Allow the muffin batter to rest at room temperature for 30 minutes.
While the batter rests, reduce and preheat the oven to 375°F / 190°C.
Prepare the cinnamon pecan streusel by combining the chopped pecans, light brown sugar, ground cinnamon, salt, and melted butter in a mixing bowl.
Stir until the streusel mixture is evenly moistened and combined.
Transfer the streusel to the refrigerator until ready to use.
Line two 12-count muffin pans with cupcake liners.
Using a 3 tbsp cookie scoop, portion the muffin batter into the prepared liners, filling every other cup if using two muffin pans.
Sprinkle the cinnamon pecan streusel evenly over the muffin batter.
Gently press the streusel into the tops of the muffins to help it adhere.
Bake at 375°F / 190°C for 18 to 22 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
The muffins are also done when the centers spring back quickly when gently pressed.
Remove the muffin pans from the oven.
Allow the muffins to cool in the pans for 5 to 10 minutes.
Carefully transfer the muffins to a wire rack.
Cool to room temperature before serving, or serve slightly warm if desired.