Melt butter in a saucepan over low heat. Add chocolate to butter; cook and stir until chocolate melts completely into the butter and the color is consistent, about 5 minutes.
Slowly stream hot water into the chocolate mixture while stirring continually; add sugar and corn syrup. Continue to cook and stir until the sugar dissolves completely into the liquid, about 10 minutes.
Remove saucepan from heat and stir vanilla extract and salt into the sauce.