In a food processor, grind cereal into a fine powder. In a medium bowl, combine flour, cocoa powder, ground cereal, cream of tartar, baking soda, and salt. In a large bowl, using an electric mixer, beat butter and sugar on medium speed until light and fluffy, about 2 minutes. Scrape down side of bowl. Add eggs and vanilla and beat to combine. With mixer on low, gradually add flour mixture and beat until combined. Chill dough for 1-2 hours.