Scrape the chocolate, add one-half of the milk and ½ cup of sugar and dissolve over a slow fire. Remove from fire, add the remaining ½ cup of milk, set aside to cool.
Sift flour once, add soda and sift three times. Cream butter and 1 cup of the sugar well, and stir in the well beaten yolks. Whip the whites of the eggs to a foam, add the cream of tartar, beat until very stiff and fold in; add the chocolate mixture and lastly the flour and stir very hard. Place at once in an ungreased patent angel cake tin or in a spring form. Put in a slow oven and bake from 40 to 60 minutes. Invert the pan and when cool remove from pan.