With a hand-held mixer, beat the egg whites in a medium bowl, until frothy. While still beating, slowly add the sugar and beat on high speed until the egg whites are stiff and shiny, about 4 minutes. (This can also be done in the bowl of an electric mixer fitted with the whisk attachment).

In another bowl, whisk the yogurt until smooth. Fold in the egg whites. Transfer the mixture to an ice cream freezer and freeze according to the manufacturers instructions. Freeze in a quart container until ready to serve.

To add chocolate stracciatella, about 2 minutes before the churning is done, melt your chocolate in a bowl in the microwave (30 seconds, stir, another 30 seconds, stir, and repeat if necessary until totally and nicely melted down). Once the chocolate is in liquid form, pour a very thin stream into the churning ice cream maker. The thin stream of chocolate will resolidify into little chunks on contact with the cold yogurt and it will be quite nicely dispersed throughout the frozen yogurt.