Bring cream, syrup, and salt to a boil in a large saucepan over medium-high heat. Remove from heat and stir in chocolate, sour cream, and butter. Let sit, stirring occasionally, until chocolate and butter are melted and mixture is smooth and no streaks of white remain. Transfer to a medium bowl. Let frosting sit at room temperature, stirring every 30 minutes or so, until thickened and spreadable (it will look like a broken chocolate sauce at first, but magically, about 1½ hours later, it will start to thicken). Stir until smooth. (If your kitchen is warmer than 72°, chill frosting 5 minutes. Stir to combine cooler outer edges with warmer center, then chill another 5 minutes. It should be the consistency of sour cream; if not, chill another 5 minutes and stir.)
https://www.bonappetit.com/recipe/blackout-cake