In a medium saucepan, whisk together the sugar, cornstarch, and salt. Gradually whisk in the milk and egg yolks until smooth, making sure there are no lumps.
Place the saucepan over medium heat and cook, whisking constantly, for about 5–7 minutes, until the mixture begins to simmer and thickens to a pudding-like consistency. Keep whisking to prevent the bottom from scorching.
Remove the saucepan from the heat. Add the chopped bittersweet chocolate, butter, and vanilla extract. Stir until the chocolate and butter melt completely and the pudding becomes smooth and glossy.
Divide the warm chocolate pudding evenly among six clear dessert cups. Allow it to cool slightly, then refrigerate for about 1 hour, or until thoroughly chilled and set.
Place the chocolate sandwich cookies in a food processor and pulse into fine crumbs. Alternatively, place them in a sealed bag and crush them with a rolling pin. Sprinkle a generous layer of cookie crumbs over each chilled pudding cup.
Use the edible black marker to write “RIP” on each vanilla sandwich cookie. Gently press one cookie into the crumbs in each cup so it stands upright like a tombstone.
Add gummy worms, candy pumpkins, or other Halloween candies to each cup. Partially bury the decorations in the cookie crumbs to create a spooky graveyard effect. Keep refrigerated until ready to serve.
