In a medium pot, combine 4 tablespoons butter and brown sugar. Place over medium heat until butter is melted.
Whisk in sweetened condensed milk and cook over medium heat, stirring constantly, until mixture bubbles. Continue cooking, stirring, for 5 minutes or until thickened and slightly darker.
Pour hot caramel into a large mixing bowl and add salt, cinnamon and vanilla (and cayenne) and beat on medium speed about 5 minutes until cooled somewhat.
Beat in one tablespoon of butter until combined, then another.
Beat another 5 minutes until only slightly warm. Beat in remaining tablespoons of butter.
Beat 5 minutes more.
Sift in powdered sugar and beat mixture until cooled.