In a heavy-bottomed saucepan, melt the butter, chocolate and corn syrup. Stir over medium-low heat until chocolate, butter and corn syrup are combined and smooth.
Add cream, then pour the sugar in the middle of the pan trying to avoid the sides. Stir carefully, trying to keep the sugar from the edges of the pan. Stir until the sugar is dissolved.
Once the sugar is dissolved, give it a final stir and then stop stirring and increase the heat to medium. Bring the mixture to a boil | DO NOT STIR!
Allow to boil for about 7-8 minutes until thick and smooth. Remove from heart and stir in the vanilla and the salt. Serve warm over ice cream. If saving, allow to cool completely before jarring and refrigerating. Rewarm in microwave safe jar in 30 second increments or set in pan of hot (not boiling) water until warmed through.
Notes
If it becomes grainy, heat in the microwave adding a small amount of heavy cream; stirring to combine until smooth. Store in refrigerator for up to a month.