*Give yourself at least a day to make this, as there are a couple of waiting periods, and freezing times.
*Makes about 2 quarts
Pour the milk into a medium-sized sauce pot and over medium heat, scald it. Tiny bubbles will appear on the edges of the pot and it should be very hot, but not boiling. Then add the semisweet chocolate and use a wooden spoon to stir. Once it's completely melted into the milk, remove the pot from the heat. Let this cool to room temperature, about 1 1/2 hours.
In a medium sized mixing bowl, use an electric mixer to whip the egg yolks with the sugar. Mix until it's very thick and pale yellow. When you drizzle it with a spoon it should rest for a second or two before sinking back into the rest of the mixture -- this is called the "ribbon stage."
On a low speed, gradually pour in the cooled chocolate mixture and blend only until it's evenly combined. Pour this back into the sauce pot and over medium-low heat, stirring almost constantly with a wooden spoon, cook until becomes a beautiful, thick custard, 8 to 10 minutes. Remove from the heat immediately and let it cool completely, at least 2 hours. (If you want to do a step a day ahead of time, you can do this step -- just let it come to room temperature, cover it tightly with plastic wrap, and refrigerate it overnight.)
Gently mix in the bittersweet chocolate, transfer to an ice-cream maker, and freeze according to the machine instructions. (While you can finely chop the chocolate with a knife, for large quantities I like to do it in my food processor with the grater attachment, to get these superfine pieces.)
Freeze for about 4 hours, or have it soft right away!