Cook’s Illustrated note:
Frosting can be made with milk, semisweet, or bittersweet chocolate. Cool chocolate between 85-100 degrees before adding it to the butter mixture. The frosting can be made 3 hours in advance.
For longer storage, refrigerate the frosting, covered, and let it stand at room temperature for 1 hour before using.
20 TBS (2 1/2 sticks or about 10 oz) unsalted butter, softened
4 oz. confectioner’s sugar
3/4 cup dutch-processed cocoa
pinch of fine table salt
3/4 cup light corn syrup or (Lyles Golden Refiner’s Syrup and 1 TBS of water)—I prefer the Lyles Golden Syrup)
1 tsp vanilla extract
8 oz chocolate, melted and cooled
In a food processor, process butter, sugar, cocoa, and salt until smooth, about 30 seconds.
Scrape the bowl as needed.
Add the syrup and vanilla and process until just combined, about 5-10 seconds.
Scrape down the bowl, and then add the chocolate and pulse until smooth and creamy, about 10-15 seconds.
Frosting can be used immediately or held.
Cook’s Illustrated doesn’t give a plain vanilla version, but I have successfully made one by replacing the cocoa powder with 1/2 cup more of confectioner’s sugar, increasing the butter by 1 TBS, and adding more vanilla extract (or whatever other extract or flavoring that you want).
The big key to this recipe is using a light hand with the processor and not over mixing it. I have also made this in a mixer, but I just watched the butter very carefully; you don’t want to whip too much air into the butter. The whole point of the processor method is being able to mix it up quickly.
Personally, I prefer to make this over whipped ganache, as ganache can be finicky and it can easily curdle from overwhipping