Whisk butter and cocoa powder in a bowl until smooth. It will take some elbow grease but keep going and it will eventually become smooth. Alternatively, you can use a mixer.
Add vanilla and sea salt, mix.
Add powdered sugar, mix well. Shake into bowl what you what you can from whisk and switch over to spatula.
Add milk, few small drops at a time until reach a creamy consistency. Mix well to get out any lumps and stop when smooth and creamy.