Melt chocolate in heatproof bowl set over saucepan containing 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and set aside. Meanwhile, heat butter in small saucepan over medium-low heat until melted. Increase heat to medium; add sugar, corn syrup, vanilla, and salt and stir with heatproof rubber spatula until sugar is dissolved, 4 to 5 minutes. Add melted chocolate, butter mixture, and cream to clean bowl of standing mixer and stir to thoroughly combine.
5. Place mixer bowl over ice bath and stir mixture constantly with rubber spatula until frosting is thick and just beginning to harden against sides of bowl, 1 to 2 minutes (frosting should be 70 degrees). Place bowl on standing mixer fitted with paddle attachment and beat on medium-high speed until frosting is light and fluffy, 1 to 2 minutes. Stir with rubber spatula until completely smooth.