Place butter, brown sugar and salt in a saucepan. Heat, stirring until the brown sugar is well dissolved. Add milk and continue stirring until blended. Let bubble (at an easy boil) for approximately 4 minutes. Stir constantly to avoid sticking. Set hot mixture aside to cool for several minutes. Using mixer, add confectioner's sugar and vanilla. You will see it turn lighter and caramelize. When you've reached the desired consistency, ice the cake, placing some of the icing between the layers.
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