Begin by preparing your cupcakes according to the package directions. Allow to cool.
Next, you'll want to make your trees. Line a baking sheet with waxed paper. Then, in a double broiler (or microwave), melt the chocolate discs. Once the chocolate is completely melted, transfer it to a plastic piping bag (or plastic baggie) and snip off a small piece from the end / corner. On the wax paper, pipe out your trees. You'll want to make them about 2 to 3 inches tall. Also, be sure to make them thick enough so that they won't break when hardened. Before the chocolate sets, add sprinkles to look like leaves. Chill in the refrigerator until the chocolate has hardened.
In the meantime, prepare your frosting.
In a double boiler, combine the sugar, water, egg whites, and cream of tartar. Cook, beating constantly with the electric mixer on high speed for about 10 - 13 minutes. (The frosting should read 160 degrees F on a candy thermometer.) Stop occasionally to scrape down the sides and bottom of the bowl to prevent sticking.
Remove the pan from the heat and add vanilla; then continue to beat for about one more minute or until the frosting is fluffy and will hold soft to stiff peaks.
Add a base coat of frosting to each cupcake. Then, using a pastry bag fitted with a large round tip, pipe the frosting in one mound on top of each cupcake. Use two dark colored sprinkles for the eyes.
Then, carefully peel the trees from the waxed paper and press into each cupcake behind the ghost.