Preheat the oven to 375°F and grease a standard 12-cup muffin pan. Set aside.
In a mixing bowl, whisk together the flour, cinnamon, pumpkin pie spice, baking powder, and sea salt. Reserve for later.
In a separate large bowl, blend the pumpkin puree with the melted butter until the mixture is smooth.
Incorporate both the granulated sugar and brown sugar, whisking until no lumps remain and the sugars are fully integrated.
Mix in the eggs and vanilla extract, stirring until well combined.
Gradually add the dry ingredients to the wet mixture, stirring until a smooth batter forms.
Fold in any desired mix-ins until evenly distributed throughout the batter.
Portion approximately ¼ cup of batter into each well of the prepared muffin tin, smoothing the tops as needed.
Bake for 20–25 minutes, or until the tops are set and a cake tester inserted into the center comes out clean or with just a few crumbs.
Allow muffins to rest in the pan for 10 minutes on a wire rack, then remove and let them cool completely on the rack.