Cream butter, oil, powdered sugar, 1/2 cup granulated sugar and the egg in large bowl until light and fluffy.
Combine the flour, cocoa, baking soda, cream of tartar and salt in small bowl.
Add to creamed mixture with vanilla, stirring until dough is smooth.
Cover; refrigerate 30 minutes or overnight, if desired.
Preheat oven to 350°F.
Lightly grease cookie sheets or line with parchment paper.
Shape dough into balls the size of marbles.
Place 2 inches apart on prepared cookie sheets.
Flatten each cookie to about 1/3 inch thickness with bottom of glass dipped into granulated sugar.
Bake 10 minutes or until firm.
Do not overbake.
Remove to wire racks to cool.