Preheat oven to 350 degrees. In a medium bowl, whisk together flour, cocoa, and salt; set aside.
In a large bowl, with an electric mixer, beat butter and sugar until light and fluffy; beat in egg. On low speed, mix in flour mixture. Divide dough in half, and form into 2 disks, each about 3/4 inch thick. Wrap disks in plastic; chill until firm, at least 45 minutes and up to 2 days.
On a lightly floured sheet of waxed paper, using a floured rolling pin, roll dough 1/4 inch thick. (If dough becomes too soft to work with, refrigerate for a few minutes.)
Using 2- to 3-inch Halloween cookie cutters, cut out shapes, dipping cutters in flour as necessary to prevent sticking; place on a baking sheet, spacing them 1 inch apart.
Bake until surface is dry to the touch, 10 to 15 minutes (if decorating, press candies into dough halfway through baking). Cool cookies 1 to 2 minutes on baking sheet; transfer to a rack to cool completely. Serve, or store in an airtight container up to 3 days.