1. In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar. Stir in vanilla. Sift together flour and cornstarch, and stir this mixture thoroughly into the creamed mixture. Stir in grated chocolate. Place dough on a sheet of plastic wrap, and form the dough into a flat disk. Wrap the dough, and chill it for at least 2 hours.

2. Heat the oven to 325 degrees. Line a baking sheet with a Silpat baking mat or parchment paper. Roll out the dough on a lightly floured surface to 1/8 inch thick. Trim to a large rectangle. Using a pizza cutter, cut into smaller rectangles approximately 1 by 2 1/2 inches in size. Chill until firm, about 20 minutes. Bake until firm and just golden, 18 to 20 minutes. Transfer tombstones to a wire rack, and cool. Decorate, as desired.