Using a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed for 2–3 minutes or until nice and creamy. Scrape down the sides of the bowl as needed.
Add in the powdered sugar and cocoa powder. Mix on low until just incorporated. It will be very thick! Again, scrape down the sides. Add in the heavy cream, vanilla, and salt. Mix again on low until incorporated.
Increase the speed to high and beat for 30–60 seconds until the frosting slightly lightens in color and becomes very creamy. If it’s too thin, add a bit more powdered sugar; if it’s too thick, mix in a little more heavy cream. Taste and adjust to your liking, adding a pinch of salt if it’s too sweet. Use immediately.
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Notes
Note 1: For the best chocolate flavor, use Dutch-process cocoa powder like Hershey’s Special Dark.Storage: Store in an airtight container in the fridge for up to 1 week. Let it sit at room temperature for 30–60 minutes to soften before using.