Bake the cupcakes & prepare the frosting. Allow the cupcakes to cool completely before frosting.
Frost the cupcakes & melt the chocolate chips. Frost all of the cooled cupcakes, leaving a little space around the rim for the ghost skirt. In a double boiler or candy melter, melt the chocolate chips, stirring frequently, until smooth. You can also melt them in the microwave, stopping every 30 seconds to stir until completely melted.
Dip the marshmallows & top the cupcakes. Dip the bottom of each marshmallow in the melted chocolate and place it on top of each frosted cupcake. Pop them into the refrigerator until the chocolate is set and hardened. You can do this step a day in advance and refrigerate them overnight.
Make the ghosts. Sprinkle the cornstarch onto a clean surface, use a baker's mat, and spread to lightly coat the surface. Slice the fondant in half, lengthwise, and lightly knead until soft. Working one half at a time, roll the fondant until about ⅛" thick. Using a 4" round pastry cutter, cut out a circle with a sharp knife.
Stretch the fondant a little, if needed, with your hands like you are working with homemade pizza dough. Now gently place on top of the cupcake, covering the marshmallow and mold to your desired ghost form. Repeat with the remaining fondant until all ghosts are made. Draw the eyes with the black edible marker.
Reroll the fondant, using all the scraps, so none goes to waste.