In a microwave safe bowl, melt the butter completely.
Add in the cocoa powder and use an electric hand mixer to combine thoroughly.
Turn off the mixer and add in the powdered sugar 1 cup at a time, adding in a little bit of heavy cream, as needed, to incorporate everything.
Repeat this process until all of the powdered sugar has been added.
Add in the vanilla extract and mix.
Check the thickness of your frosting and adjust by adding more heavy whipping cream, if desired.
Once mixed, spread onto your brownies. The frosting will set up as it cools.
Enjoy!