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