Combine powdered sugar and butter in a stand mixer fitted with the paddle attachment. Beat on medium speed until light and fluffy, about 5 minutes. Add cocoa powder, vanilla, and salt; mix for 30 seconds. Add cream 1 tablespoon at a time, mixing after each addition. Scrape down the sides of the bowl. Mix on medium speed until frosting is shiny and seems to have large bubbles, 3 to 5 minutes more.
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