Separate eggs. Allow egg whites and yolks to stand at room temperature for 30 minutes.
Preheat oven to 325 degrees F. In a medium mixing bowl beat egg yolks with an electric mixer on high speed about 5 minutes or until thick and lemon colored. Add milk and vanilla; beat on low speed until combined. Gradually beat in the 1 cup sugar at low speed. Increase to medium speed; beat until mixture thickens slightly and doubles in volume (about 5 minutes total).
In a small bowl combine flour and cocoa. Sprinkle 1/4 cup of the flour mixture over egg yolk mixture; fold in until combined. Repeat with remaining flour mixture, 1/4 cup at a time. Set egg yolk mixture aside.
Thoroughly wash beaters. In a large mixing bowl beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually add the 1/2 cup sugar, beating on high speed until stiff peaks form. Fold 1 cup of the beaten egg white mixture into the egg yolk mixture; fold egg yolk mixture into remaining egg white mixture. Pour into an ungreased 10-inch tube pan.
Bake for 55 to 60 minutes or until cake springs back when lightly touched. Immediately invert cake; cool thoroughly in pan. Loosen sides of cake from pan; remove from pan. If desired, top your cake with chocolate ganache, icing, or whipped cream.