Cut chocolate into small bits. Heat cream over medium heat and add chocolate, sugar and egg yolks, stirring constantly. When thickened, add vanilla and bourbon, stirring constantly until smooth. Pour through a sieve into a metal container for freezing. Let cool, cover and freeze for 1 hour. In 1 hour, stir to break up any ice crystals that have formed and recover and freeze. To serve, place in the microwave for 30 seconds and scoop out into serving dishes.