Preheat oven to 350 degrees. Line 12 muffin cups with paper liners, set aside. In a small mixing bowl whisk together, flour, baking powder, baking soda, salt, cinnamon, ginger and nutmeg, set aside. In the bowl of an electric mixer, whip together butter and brown sugar until lighter and slightly fluffy about 3 - 4 minutes. Stir in egg and vanilla. Slowly add in dry ingredients alternating with buttermilk. Stir until well combine. Mix in sweet potato puree. Fill muffin cups 2/3 full (about scant 1/4 cup in each). Sprinkle 1/2 tsp brown sugar over each cupcake.
Bake 19 - 21 minutes until toothpick inserted in center comes out clean. Cool 5 minutes in muffin tins before removing to wire rack to cool. Cool cupcakes on wire rack at least 5 minutes. Generously butter a cookie sheet. Spread mini marshmallows in a single layer on cookie sheet, keeping the marshmallows close together. Heat oven to broil. Place marshmallows in oven under broiler (DO NOT WALK AWAY FROM THEM), cook about 30 seconds-1 minute until they start to brown slightly. Remove from oven and using a buttered spatula scoop marshmallows over tops off cupcakes.
*I use fresh sweet potatoes in mine:
To do this preheat oven to 400 degrees. Poke 1 medium sweet potato several times, place on a baking sheet in the oven and bake until soft about 1 hour. Allow to cool slightly, then peel and puree potato in a food processor, or mash. You can also use canned pureed sweet potatoes if you'd like.