In a food processor, pulse the flour, cocoa and salt. Add the butter and process until sandy, about 3 minutes. Add the confectioners' sugar, milk and vanilla extract and process until a firm dough forms. Transfer the dough to 2 sheets of plastic wrap and form into two 7-inch logs, about 1 1/2 inches thick. Wrap in plastic and refrigerate until very firm, at least 4 hours or overnight.