Preheat the oven to 375°F.
Set aside a 12-cup muffin tray that has been greased.
Combine the flour, pumpkin pie spice, cinnamon, baking powder, and sea salt in a small mixing basin. Place aside.
In a large mixing basin, combine pumpkin puree and melted butter until smooth.
Whisk in the brown sugar and granulated sugar until mixed and there are no lumps.
Stir in the eggs and vanilla extract.
Stir in the dry ingredients until the batter is smooth.
If desired, whisk in mix-ins such as chocolate chips, cinnamon chips, walnuts, and so on until equally distributed.
1/4 cup pumpkin muffin batter should be poured into each muffin tray thoroughly and smoothed out.
Bake for 14-17 minutes in a preheated oven, or until the tops are set and a cake tester put into the middle of the loaf comes out clean or with a few crumbs. (I bake mine for about 15 minutes.)
Allow pumpkin muffins to cool for 10 minutes in the pan on a wire rack before removing and transferring them to a wire rack to cool entirely.