Combine softened butter, cocoa powder and salt in a large mixing bowl. Add in 1/2 of powdered sugar. Beat on medium heat with an electric mixer until just combined.
Add in 1/4 cup of remaining sugar. Beat again. Add final 1/4 cup and beat till fully combined.
Stir in cream and vanilla. Mix on medium high speed until fluffy and desired texture has been reached.
Notes
To make ahead of time, just pour into an airtight container and keep in the fridge. When ready to use, bring frosting to room temperature.