This is the frosting recipes that goes with the Old fashioned Chocolate Layer Cake from Cook's Illustrated.
454g semisweet chocolate
115g unsalted butter
1/3 cup sugar
2 tsp vanilla extract
¼ tsp table salt
1 ¼ cup whipping cream (cold)
I omit the corn syrup.
Melt semisweet chocolate, finely chopped, in a heat-proof bowl set over saucepan containing 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and set aside.
Meanwhile, heat unsalted butter in small saucepan over medium-low heat until melted. Increase heat to medium; add sugar, vanilla extract, and salt and stir with heat-proof rubber spatula until sugar is dissolved, 4 to 5 minutes. Add melted chocolate, butter mixture and heavy cream to clean bowl of standing mixer and stir to thoroughly combine.
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. 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.
Note: Do not substitute semisweet chocolate chips for the chopped semisweet chocolate in the frosting.
Chocolate chips contain less cocoa butter than bar chocolate and will not melt as readily. For best results, don't make the frosting until the layers are cooled, and use the frosting as soon as it is ready. If the frosting gets too cold and stiff to spread easily, wrap the mixer bowl with a towel soaked in hot water and mix on low speed until the frosting appears creamy and smooth. Refrigerated leftover cake should sit at room temperature before serving until the frosting softens.