Preheat oven to 200 degrees F and line a baking sheet with parchment paper. Set it aside while you prepare the meringue mixture.
Pour egg whites into a large bowl and start beating them on low speed using an electric mixer. Gradually add cream of tartar, then slowly incorporate sugar while increasing the mixer speed to its highest setting until you achieve bright white semi-stiff peaks.
Scoop the meringue mixture into a piping bag fitted with Wilton tip #12. Pipe out mounds on the prepared baking sheet, creating bases about 1 inch in diameter and reaching up to about 2 inches high, pulling back at the top to form pointed tips.
Bake for 2 hours, then turn off the oven but leave the meringues inside for an additional 30 minutes to finish drying out completely.
As your meringue bakes, mix together softened unsalted butter, confectioners' sugar, and milk in another bowl using an electric mixer until smooth and creamy. Add a drop of purple gel food coloring and mix until fully tinted.
Once cooled, frost each cupcake with your purple buttercream using a piping bag fitted with Wilton tip #12. Top each cupcake with one meringue ghost and decorate with black frosting for eyes and mouth as desired.