Preheat oven to 400 degrees . In a large bowl, combine the sugar and butter; beat until light and fluffy. Add vanilla and eggs and blend well. Add flour, cocoa, cream of tartar, baking soda, and salt, and mix well. In a small bowl combine 2 tablespoons sugar and cinnamon. Shape dough into 1-inch balls and roll in the sugar-cinnamon mixture. Place 2 inches apart on ungreased cookie sheets. Bake until set, about 8 to 10 minutes. Immediately remove from cookie sheets and cool.