Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).

Grease a standard 12-cup muffin tin, set aside.

Combine flour, pumpkin pie spice, cinnamon, baking powder and sea salt in a small bowl. Then, set it aside.

Whisk the pumpkin puree and melted butter in a large bowl until smooth.

Add brown sugar and granulated sugar and whisk until the sugars are dissolved.

Then, whisk in eggs and vanilla.

Add the dry ingredient mixture and whisk or stir until batter is smooth.

If desired, add mix-ins like chocolate chips, cinnamon chips, walnuts, etc. and stir until evenly distributed.

Pour ¼ cup of the pumpkin muffin batter into each well of the muffin tin, and smooth out the top.

Bake in preheated oven for 20-25 minutes, or until the tops are set and a cake tester inserted into the center comes out clean.

Let pumpkin muffins cool in the pan on a wire rack for 10 minutes, then transfer them to a wire rack to cool completely.