In a small heavy-bottomed saucepan, bring sugar, 1 cup water, corn syrup, and cream of tartar to a boil over high heat. Cook, stirring frequently, until mixture is amber in color and registers 240°F on a candy thermometer. Remove from heat. Add butter, whisking constantly until melted. Add cream, and whisk to combine. (The mixture will foam.) Stir in bourbon, vanilla, and salt until combined. Let cool completely. Store, covered, at room temperature for up to 1 week.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/guinness-stout-cake/