Add vinegar to milk, set aside
Thoroughly cream shortening and sugar
Add egg, extract, and soured milk to mixture and combine
Sift together flour, cocoa, soda, and salt; add to creamed mixture, blend until well combined
Shape dough into roll, about 2 inch in diameter,
Wrap dough in waxed paper, freeze for up to 3 weeks or chill for 4-6 hours
Slice dough into thin cookies and place on ungreased cookie sheet
Bake at 350 degrees for 6-8 minutes