Beat butter at medium speed with an electric mixer until fluffy; gradually add sugars, beating well. Add eggs, 1 at a time, beating after each addition. Add melted chocolate and vanilla, mixing well.
Combine flour, baking soda, and salt; add to batter alternately with buttermilk, beginning and ending with flour. Mix at low speed after each addition until blended.
Spoon batter into paper-lined standard muffin pans, filling each cup full. Bake at 350° for 15 to 18 minutes or until a wooden pick inserted in center comes out clean. Cool 5 minutes in pans. Remove from pans, and cool completely on a wire rack.
Take a plug out of center top of each cupcake, going pretty deep but not quite to the bottom of each cake. (Reserve these little cupcake pieces.)
Spoon marshmallow crème into a zip-top freezer bag; seal bag, and cut a hole (about 1/2") in 1 corner. Squirt crème into hole of each cupcake. Replace just the top portion of each cupcake piece to regain a smooth top. Gently frost cupcakes; decorate as desired.
*We tested with peppermint candies and a Winter Wonderland decorating kit from Williams-Sonoma.