Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease the cups.
In a large mixing bowl, whisk together the all-purpose flour (or oat flour), protein powder, baking powder, baking soda, pumpkin pie spice, and salt.
In a separate bowl, whisk the pumpkin puree, maple syrup (or honey), milk, vanilla extract, and eggs until smooth.
Pour the wet mixture into the dry ingredients. Stir gently until just combined. Do not overmix.
Fold in any desired optional add-ins like chocolate chips or chopped nuts.
Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely.