In a saucepan combine milk, cream, and vanilla bean.
Simmer over low heat for 30 minutes.
In another bowl, combine sugar, cocoa, eggs, and yolk until thick and pale yellow.
Measure approximately 1 cup from the hot cream; slowly add to chocolate mixture while stirring constantly.
Stir chopped chocolate into the saucepan as well as the hot egg / chocolate mixture.
Cook over low, stirring constantly until mixture coats the back of a spoon.
Transfer to a bowl and cover; refrigerate until completely cooled.
Turn ice cream machine on.
Pour in chilled custard and mix, about 25-30 minutes until desired consistency.