Bring some water to a boil in a saucepan, lower to a simmer.
Place a heat proof bowl over (not on) the water and add your chocolate and butter to the bowl.
In a separate saucepan warm your cream over low heat.
Now add your sugar and corn syrup to the cream, stir this until the sugar dissolves. Must be completely dissolved before moving to next step.
Now add the warm cream mixture to the melted chocolate mixture.
Keep over low heat and stir constantly for about 10 minutes until it starts to thicken.
Add the vanilla.