FOR THE CUPCAKES: Move oven racks to lower-middle and upper-middle positions. Preheat oven to 350 degrees F. Line 2 muffin tins with cupcake liners; set aside.
In a large bowl, add flour, baking powder, baking soda, salt, pumpkin pie spice, cinnamon, and nutmeg. Whisk until combined; set aside. In a medium bowl, whisk together brown sugar, eggs pumpkin, oil, milk, and vanilla until combined. Pour wet ingredients into dry ingredients and whisk until just combined and no flour streaks remain. Do NOT overmix the batter.
Using a 4 Tablespoon scoop, scoop batter into the prepared cupcake cups (they should be 2/3 full). Bake for 18-20 minutes, rotating pans half-way through baking. When a toothpick is inserted in center it should come out clean. Cool 10 minutes in pans, then move cupcakes to a wire rack to cool completely before frosting. This should take about 1 hour.
FOR THE FROSTING: Using stand mixer fitted with a paddle attachment, or with handheld mixer, beat butter, marshmallow fluff, vanilla, and cream on medium speed until completely smooth and fluffy, about 3-4 minutes. Add powdered sugar and salt. Pulse mixer until sugar is absorbed, about 1 minute. Then increase speed to medium-high and beat for 2-3 minutes.
TO FROST: Fit a 14 or 16-inch disposable piping bag with a Wilton #1M piping tip (cut tip off piping bag so just ½ of the piping tip shows). Full with buttercream. Frost cupcakes by starting on outside edge of cupcake and swirling frosting to make a mound. Garnish with orange nonpareils and pumpkin candy corn (optional). Serve.