Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or spray it lightly with a non-stick cooking spray.

In a large mixing bowl, whisk together the pumpkin puree, Greek yogurt, eggs, maple syrup, and vanilla extract until the mixture is smooth.

In a medium bowl, combine the oat flour, protein powder, pumpkin pie spice, baking powder, baking soda, and salt. Stir well to remove any lumps.

Slowly add the dry ingredients to the wet ingredients. Use a spatula to fold them together gently, stopping as soon as no more dry flour streaks are visible. If you are adding chocolate chips or nuts, fold them in now.

Divide the batter evenly among the 12 muffin cups, filling them about ¾ of the way full.

Bake for 18 to 22 minutes. To check if they are done, insert a toothpick into the center of a muffin; it should come out clean or with just a few moist crumbs.

Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.