Heat the oven to 350°F (175°C). Line a 12-cup muffin pan with paper liners or lightly grease each cavity.

Place 1 1/2 cups of the rolled oats in a blender or food processor and pulse until they resemble coarse flour. Keep the remaining 1/2 cup whole for texture.

Whisk the pumpkin purée, eggs, maple syrup, milk, and vanilla in a large bowl until smooth and evenly blended.

Add the ground oats, whole oats, baking powder, baking soda, cinnamon, pumpkin pie spice, and salt. Stir just until the dry ingredients are moistened, then fold in chocolate chips or nuts if using.

Let the batter stand for about 5 minutes so the oats can absorb some of the liquid. This short rest helps the mixture thicken before it goes into the muffin pan.

Divide the batter among the prepared muffin cups, filling each about three-quarters full. Try to distribute the whole oats and any mix-ins evenly from cup to cup.

Bake for 18 to 22 minutes, until the tops are set and a toothpick inserted in the center comes out clean or with a few moist crumbs. Begin checking near the 18-minute mark because ovens and pan materials can vary.

Cool the muffins in the pan for 5 minutes, then transfer them to a wire rack. Serve warm or let them cool completely before storing.