Preheat your oven to 375°F (190°C). Line a standard 12-cup muffin tin with paper liners or spray with non-stick cooking spray. For bakery-style domed tops, preheat to 425°F, then reduce to 375°F after 5 minutes of baking.
In a medium bowl, whisk together the flour, pumpkin pie spice, baking soda, and salt.
Place the raisins in a small bowl and cover with hot water for 5 minutes, then drain well and pat dry.
In a large bowl, whisk together the pumpkin puree, brown sugar, oil, eggs, milk, and vanilla extract until smooth and well combined.
Add the dry ingredients to the wet ingredients and stir just until combined. Do not overmix.
Gently fold in the drained raisins and nuts (if using) with a few strokes.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.
If desired, sprinkle turbinado sugar over the tops of the muffins.
Bake for 20-22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.