Preheat the oven to 350°F and line muffin pans with paper liners.
Whisk the flour, baking powder, baking soda, salt, cinnamon, and pumpkin pie spice together in a large bowl. Set aside.
Whisk the oil, eggs, brown sugar, granulated sugar, pumpkin, and vanilla extract together until combined. Pour the wet ingredients into the dry ingredients and use a mixer or whisk until completely combined. Batter will be thick.
Scoop batter into the prepared muffin pans ( I like to use a large cookie scoop). Bake for 20-22 minutes. Baking times vary, so keep an eye on yours. The cupcakes are done when a toothpick inserted in the center comes out clean.
Remove the cupcakes from the oven and set the entire pan on a wire rack. Allow to cool completely.
Make the frosting: In a large bowl using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the marshmallow fluff and butter together on high speed until smooth and creamy. Add confectioners' sugar, vanilla, cinnamon and salt. Beat on low speed for 30 seconds, then switch to high speed and beat for 2 minutes. If you want the frosting a little thicker, add the extra 1/4 -1 cup of confectioners sugar. Spread the frosting on the cooled cupcakes. Refrigerate for 30 minutes before serving.
Cover leftover cupcakes tightly and store in the refrigerator for 5 days.