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 3 cups of 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: The secret to the best chocolate buttercream frosting is using Dutch-process cocoa powder. The Dutching process offers a much more robust and intense chocolate flavor. Hershey’s Special Dark cocoa powder is a Dutch-process cocoa powder readily found at most grocery stores. Storage: Use a spatula to scoop all the frosting from the bowl into an air-tight container. Place in the fridge and store for up to 1 week. When ready to use, let the frosting stand at room temperature for about 30–60 minutes to soften to spreadable consistency.