PREPARATION:

1. In a small saucepan, whisk together sugar, cocoa, flour and salt. Gradually whisk in milk. Place over medium-high heat and bring to a boil, whisking constantly, until a thick custard forms, about 1 to 2 minutes. (The custard can burn quickly, so whisk thoroughly and constantly.) Remove from heat. Stir in chocolate chips until melted and smooth. Pour custard into a bowl and cover with plastic wrap; plastic wrap should touch the surface of the custard to prevent skin from forming. Refrigerate until cold, or place bowl in larger bowl of ice and stir occasionally, being careful not to get water in the custard.
2. Add whipped topping to custard and whisk just until combined. Spoon custard into crusts (about 1/4 cup per crust). Cover tarts with plastic wrap touching the custard and refrigerate at least 2 hours or overnight.
3. Before serving, form a ghost on top of the custard with dollops of whipped topping. Place candies on ghost to form eyes and mouth.