Sift the cocoa powder and powdered sugar together through a fine-mesh sieve into a bowl. This prevents lumps and ensures ultra-smooth frosting.
In a large mixing bowl, beat room temperature butter with mixer on medium speed for 2-3 minutes until light and fluffy. The butter should look pale and increase in volume.
Reduce mixer speed to low. Gradually add the sifted cocoa-sugar mixture, one cup at a time, beating well after each addition to prevent a powdered sugar cloud.
Add vanilla extract and salt. Beat on medium speed for 1 minute to incorporate fully.
With mixer on low speed, slowly drizzle in the cream (or plant-based alternative). Increase speed to medium-high and beat for 2-3 minutes until frosting is light, fluffy, and pipeable.
Stop mixer and scrape bowl sides. Beat for an additional 30 seconds. The frosting should hold its shape when piped but spread smoothly when applied with an offset spatula.