Heat the cream, butter and sugar in a saucepan over low heat until it has come to a boil.
Put the thermometer in the mixture and allow the heating to become 115 degrees Celsius. If you really don't own a sugar thermometer take a drop of the mixture and let it fall in a container with ice cold water. If the drop keeps it form the cooking is finished.
Turn of the heat and stir in the vanilla and continue beating until the mixture is cooled down and becomes a bit crumbly.
Pour it into the mold.
Allow to harden for at least 4 hours at room temperature. Longer is better! Cut the fudge into the desired shape.