Put the butter in the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until butter is smooth with no lumps. With the motor running, add the sugar, 1 cup at a time, only adding the next cup after the first addition has been integrated into the mixture.
Stop the machine and add the vanilla and salt. Paddle on low-medium speed until completely smooth, approximately 2 minutes. Add the water and continue to mix until light and fluffy, 2-3 minutes.
The frosting can be kept in an airtight container at room temperature for up to 2 days.