Preheat your oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners or lightly grease with oil.
In a large bowl, whisk together pumpkin purée, eggs, Greek yogurt, maple syrup, brown sugar (if using), oil, and vanilla until smooth and well combined.
In a separate bowl, stir together flour, oats, protein powder, pumpkin pie spice, cinnamon, baking powder, baking soda, and salt until evenly mixed and no clumps remain.
Add the dry ingredients to the wet ingredients. Use a spatula to fold gently until just combined. Do not overmix; a few small streaks of flour are okay.
Fold in any optional mix-ins like chocolate chips or nuts.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full. If you like, sprinkle a few oats or pumpkin seeds on top for texture.
Bake for 16–20 minutes, or until the tops spring back lightly and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. The muffins will firm up as they cool.