In the bowl of a standing mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, beat the butter, vanilla and sea salt for 1 minute.
Add 2 cups of the powered sugar, 1 cup at a time, beating thoroughly after each addition.
Then, add the milk and 2 more cups powdered sugar (you should be up to 4 cups by this time), and beat until smooth.
Add the additional cup of powdered sugar and beat until smooth and fluffy.
