Prep the pan and oven: Heat oven to 375°F (190°C). Line a 12-cup muffin tin with liners or grease lightly.
Mix dry ingredients: In a large bowl, whisk 2 cups flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt, 2 teaspoons cinnamon, 1/2 teaspoon nutmeg, 1/2 teaspoon ginger, and a pinch of cloves.
Whisk wet ingredients: In a medium bowl, combine 1 cup pumpkin puree, 3/4 cup granulated sugar, 1/2 cup brown sugar, 2 large eggs, 1/2 cup oil, 1 teaspoon vanilla, 1/2 cup milk, and 1/2 cup sourdough starter until smooth.
Combine gently: Pour wet into dry.
Stir with a spatula just until the flour streaks disappear. If using add-ins, fold in up to 1 cup now. Do not overmix.
Portion: Divide batter evenly among muffin cups, filling about 3/4 full.
Sprinkle tops with coarse sugar if desired.
Bake: Bake 18–22 minutes, until the centers spring back and a toothpick comes out with a few moist crumbs.
Cool: Let muffins rest in the pan 5 minutes, then transfer to a rack to finish cooling. Enjoy warm or at room temperature.