Use an electric mixer to whisk egg whites with lemon juice in a bowl, add sugar gradually, spoon by spoon, whisk until snow white, shiny, and really thick.
Spoon meringue into a piping bag. Pipe 12 puffs resembling ghosts on each of the baking sheets. Bake for about 1 hour until firm and dry. Turn off the heat and let sit in the closed oven for 1 more hour.
Use a toothpick to draw eyes and mouth with melted chocolate.