1. Preheat oven to 400°. In a blender, whirl cocoa nibs until each is about the size of a grain of rice.
2. In the large bowl of an electric mixer, beat butter and 1/2 cup sugar until creamy; beat in egg and vanilla.
3. In another bowl, whisk together flour, baking powder, salt, and cocoa nibs; gradually add to butter mixture, blending thoroughly, to form a soft dough. Divide dough into thirds, cover each portion tightly with plastic wrap, and refrigerate until firm (at least 1 hour), or up to 3 days.
4. On a floured board, roll out dough, a portion at a time, to a thickness of 1/8 in. (keep dough refrigerated when not in use). With a sharp knife, cut out free-form tombstone shapes (about 1 1/2 by 3 in.; cut bottom edges at an angle to make them easier to poke into the pots de crème), and place slightly apart on ungreased baking sheets. Sprinkle generously with sugar.
5. Bake cookies until edges are lightly browned, about 8 to 10 minutes. Transfer to racks and let cool completely before handling.
6. Using a pastry bag with a very fine tip, pipe the letters "RIP" in chocolate on at least 8 of the cookies. Stick these cookies into the Dark Chocolate Graveyard Pots de Crème and serve the rest of the cookies alongside.
Note: Nutritional analysis is per cookie.
Cocoa nibs, also called "cacao nibs," give these cookies a mottled look that resembles stone. We made the recipe with both chocolate-covered nibs made by Scharffen Berger and plain nibs from Dagoba Organic Chocolate, and both worked well. Find both types in gourmet markets. Or substitute 2 tbsp. finely chopped bittersweet chocolate if you prefer. Make up to 3 days ahead; store airtight.