Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.

In a large bowl, combine the sourdough starter, pumpkin puree, vegetable oil, brown sugar, granulated sugar, eggs, and vanilla extract until well-blended and smooth.

In another bowl, whisk together the flour, baking soda, baking powder, cinnamon, nutmeg, and salt until evenly dispersed.

Gradually add the dry mixture to the wet mixture, stirring gently until just combined (don’t overmix—the texture should be slightly lumpy).

Divide the batter evenly among the muffin cups, filling each about 2/3 full to allow room for rising.

If using, sprinkle the crumb topping over each muffin.

Place the muffins in the oven and bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.

Let the muffins cool for a few minutes in the pan before transferring them to a wire rack to cool completely.