Sift or whisk the flour, baking powder, baking soda, salt, and pumpkin pie spice in a small bowl and set aside.
Combine the buttermilk, pumpkin puree, and vanilla in a measuring cup and whisk until smooth. Set aside.
In a large mixing bowl, cream the butter and sugar until light and fluffy, about 2-3 minutes. Scrape down the sides.
Add the eggs, one at a time, mixing well after each addition. Scrape down sides of the bowl.
With the mixer on low speed, alternate adding dry and wet, beginning and ending with dry.
Scrape sides of bowl and let batter sit for 15 minutes.
Preheat oven to 350° F. Turn mixer on low and briefly mix batter again (no more than 20 seconds or so).
Scoop batter into muffin pan lined with cupcake liners. Bake for 22-25 minutes, or until cupcakes spring back when lightly touched.
Cool in pans for 10 minutes. Remove from pans and place on wire racks to finish cooling.
Make the Maple Buttercream
Fill a small saucepan with about one inch of water and begin simmering. In the bowl of a stand mixer, add egg whites and both sugars. Place the bowl over the simmering water (the bottom should not touch the water) and whisk every 30 seconds or so. Continue whisking until the sugars have melted and you don't feel any granules of sugar when rubbing the mixture between your fingers (this will be to at least 110° F on a thermometer).
Remove bowl from simmering water and place it on mixer. Using the whisk attachment, whip over high speed until stiff, glossy peaks form. This will take anywhere from 5-10 minutes. Slowly add the butter in two tablespoon chunks until all the butter has been added. The buttercream may look soupy at this point, but just keep whipping and it will transform into a smooth, luscious frosting.
Switch to the paddle attachment and stir on low to knock out any air bubbles for 10-15 minutes. This is optional, but helps with piping or decorating.
Cupcakes can be stored at room temperature, covered, for two days.