Combine the brown sugar, salt, butter, and milk in a small pot. Heat over medium-low heat until boiling, stirring very frequently to prevent scorching. Boil for 2 minutes. Remove from the heat and cool completely. Using a stand mixer or with a hand mixer, combine the cooled caramel with the powdered sugar and beat until combined and fluffy. You may find it necessary to add a little more powdered sugar if it's too runny or a few drops of milk if it's too thick to spread.
Makes enough icing to cover a 9x13 cake.
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