Using a stand mixer fitted with a whisk attachment, beat butter on high speed for about 4-5 minutes until creamy and lightened in color. Scrape down the bowl as needed.
Add 1/3 of the confectioners sugar, cocoa powder, and salt and beat on low speed until combined. Add 1/4 cup heavy cream and vanilla extract and beat on medium speed. Reduce speed and add another 1/3 of the confectioners sugar. Beat until combined. Add the last third of confectioners sugar. Beat on low until incorporated and then increase speed to medium/high.
Continue beating for 5 minutes, scraping down the bowl several times to ensure everything is well blended. Be sure there are no clumps of butter on the bottom of the bowl. Add additional heavy cream as needed.