Combine the chocolate, heavy cream, butter, corn syrup, brown sugar, cocoa powder and salt in a small saucepan over medium heat. Cook, stirring often, until the chocolate is melted and smooth, about 5 minutes. (Do not simmer or the chocolate may scorch and taste bitter; reduce the heat if needed.)
Remove from the heat and stir in the vanilla. Let cool slightly. Refrigerate up to 1 week; reheat in a small saucepan over low heat or microwave in 30-second intervals, thinning with more heavy cream if needed.
Use milk chocolate instead of semisweet along with 1/3 cup creamy peanut butter; increase the heavy cream to 1 cup and the brown sugar to 1/4 cup. Stir in 1/2 cup chopped salted roasted peanuts at the end.
Use half semisweet and half milk chocolate along with 2/3 cup butterscotch chips; increase the butter to 6 tablespoons.
Add 1/3 cup seedless raspberry jam with the chocolate. Replace the butter with coconut oil and omit the corn syrup and brown sugar. Use coconut extract instead of vanilla.
Add 1/3 cup cherry jam with the chocolate. Omit the corn syrup and brown sugar. Use 1/2 teaspoon almond extract instead of vanilla.
Add 1/3 cup orange marmalade and 1/2 teaspoon chipotle chile powder with the chocolate. Omit the corn syrup and brown sugar. Add 1/4 teaspoon orange extract with the vanilla.
Use milk chocolate instead of semisweet along with 1/3 cup chocolate-hazelnut spread and 1 tablespoon instant espresso powder. Omit the corn syrup, brown sugar and vanilla. Stir in 2 tablespoons hazelnut liqueur at the end.