Heat the oven to 325 degrees F/160 degrees C.
Put the chocolate in a bowl. Bring the cream, milk and sugar to a boil in a saucepan. Remove from the heat and add the chopped chocolate, stirring to melt. (Be sure to give it enough time: you don't want little bits floating around in there.) Break the egg and yolk into a bowl. Gradually whisk the chocolate mixture into the egg mixture, taking care to not to scramble the eggs. Strain the mixture into the baked tart shell. Bake until set, about 20 minutes. Cool completely.