Everything at room temperature.
12 oz (1.5 packs) cream cheese
3/4 cup unsalted butter
3 1/2 cup icing sugar
1 tsp vanilla extract
2/3 cup cocoa powder
pinch salt
Cream butter and sugar until fluffy and sugar is completely absorbed
Scrape the bowl with a silicone spatula every 30 seconds
Add the rest of the ingredients, and beat until smooth.
This should have the consistency of hard peaks.
If it seems too hard, add milk or cream; beat in 1 tsp until smooth.
If it seems too wet, add more icing sugar; beat in 1 tbsp until smooth.