In a large mixing bowl, beat the butter until creamy and smooth using an electric mixer or whisk.
Gradually add the sifted powdered sugar and cocoa powder, mixing on low speed until incorporated.
Stir in the vanilla extract and salt, then gradually add the milk or heavy cream one tablespoon at a time until the frosting reaches a spreadable consistency.
Increase the mixer speed to medium-high and beat for 2-3 minutes until the frosting is fluffy and smooth.
Spread over cooled brownies with a spatula and let set before slicing and serving.
Notes
For a richer flavor, substitute part of the cocoa powder with melted dark chocolate. Store leftover frosting in an airtight container in the refrigerator for up to a week.