In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until it is smooth, about 1 minute.
Add the confectioners’ sugar and cocoa powder to the bowl and beat until combined. With the stand mixer running on low speed, slowly stream in the milk and vanilla extract then add the salt and continue beating until well combined, scraping down the sides as needed, about 2 minutes.
Increase the speed to high and beat the frosting for an additional 2 minutes. Use the frosting immediately or store in an airtight container in the fridge. (If you refrigerate the frosting, you may want to re-blend it for a few seconds in the stand mixer before using it.)
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