In a large mixing bowl, beat butter with an electric mixer on medium to high speed 30 seconds. Add the 1 cup sugar, soda, and cream of tartar. Beat until combined. Beat in egg. Beat in cocoa powder and as much of the flour as you can with the mixer. Stir in any remaining flour.

Drop by slightly rounded teaspoons 2 inches apart onto an ungreased cookie sheet. Stir together remaining sugar and cinnamon. Sprinkle dough with sugar-cinnamon mixture. Bake in a 375 degree F. oven for 9 to 11 minutes or until edges are firm. Cool on wire racks. Makes about 30 cookies.

Store cookies in airtight container at room temperature up to 3 days or in freezer up to 1 month.