Place the icing sugar, corn syrup, salt, vanilla, and hot water in a large mixing bowl or in the bowl of a stand mixer fitted with a paddle attachment. Using a whisk or with the machine on low speed, blend until the mixture is smooth and all of the sugar has been incorporated, scraping down the sides of the bowl with a spatula as necessary. Add the melted chocolate and stir to combine completely. If the icing seems too thick, add more hot water, one teaspoon at a time.