Everything at room temperature
1 cup unsalted butter
3.5 cup icing sugar
1 tsp vanilla extract
1/2 cup cocoa powder
3 tbsp cream
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
This should have the consistency of hard peaks.
If it seems too hard, add more milk; beat in 1 tsp until smooth.
If it seems too wet, add more icing sugar; beat in 1 tbsp until smooth.