Preheat & Prepare - Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners.
Mix Dry Ingredients - In a bowl, whisk together all-purpose flour, baking powder, baking soda, salt, pumpkin spice, and cinnamon.
Combine Wet Ingredients - In another bowl, whisk together pumpkin purée, vegetable oil, brown sugar, eggs, milk (or buttermilk), and vanilla extract until smooth.
Fold Together - Gradually add the dry ingredients to the wet ingredients and fold until just combined. Do not overmix.
Add Mix-Ins - Fold in optional mix-ins such as nuts, chocolate chips, or cream cheese swirl whatever you like.
Fill Muffin Cups - Divide the batter evenly among the 12 muffin cups.
Bake - Bake for 20-22 minutes or until a toothpick inserted in the center comes out clean.
Cool -- Let muffins cool in the pan for 5 minutes, then transfer to a wire rack.
Serve - Enjoy warm or at room temperature.