Preheat the oven to 400°F (205°C). Line a baking sheet with foil and spray it with cooking spray. Pierce the potatoes several times with the tines of a fork or the tip of pairing knife. Bake 45-60 minutes until a knife slides easily into and through the potatoes. The cooking time will vary based on the size of the potatoes.

Remove the potatoes from the oven and let them cool until you can handle them with your hands. Cut the potatoes in half and scoop out the filling into a bowl and let the sweet potatoes cool to room temperature.

Puree sweet potatoes in a food processor until it is smooth, about 20 seconds. Measure out 300 grams. Use any leftover sweet potatoes in pancakes, or a frittata.

Place the pecans, brown sugar, ground cinnamon, salt, and soft butter in a bowl and use and fork to mash the ingredients together until it starts to form a crumble.

Set the streusel aside until ready to place the muffins in the oven.

Preheat the oven to 425°F (220°C). Line a 12-cup muffin pan with paper liners.

Melt the butter in a small saucepan over medium-low heat and then let it cool to room temperature. If the butter starts to splatter, reduce the heat to low.

Place the brown sugar, cinnamon, and nutmeg in a bowl and whisk together. Whisk in the melted butter, then add the eggs one at a time and whisk until each egg is thoroughly incorporated. Whisk in the sweet potato puree and sour cream.

Let the wet ingredients sit for 10 minutes to give time for the sugar to dissolve.

Add half of the flour mixture to the wet ingredients. Use a spatula to fold the dry ingredients into the wet ingredients. Add the remaining flour mixture and fold it in until the dry ingredients are thoroughly incorporated, and no white specs of flour remain. The batter may be a little lumpy which is okay.

Divide the batter between the 12 muffin cups. The batter should almost be the top of each paper liner. You will need to thoroughly scrape the bowl to completely fill the muffin cups. Top each muffin with the pecan streusel and lightly press the streusel into the batter.

Bake the muffins at 425°F (220°C) for 5 minutes, then without opening the oven door reduce the temperature to 350°F (175°C) and bake the cupcakes until the center is done and the temperature in the center of the muffin registers at 202°F (95°C), about 15-18 more minutes.

Remove the muffins from the oven and let the muffins cool in the muffin pan for 5 minutes. Place the individual muffins on a wire rack to finish cooling. The muffins can be served warm.

Store the pumpkin muffins in an airtight container for up to 3 days, and then store in the refrigerator for up to a week. The muffins can also be individually wrapped and frozen. When ready to eat reheat the frozen muffin in the oven.