Prepare brownies according to package directions and allow to cool completely before beginning icing.
To make icing, combine sugar, milk, shortening, margarine & salt in heavy saucepot on stovetop.
Bring to a rolling boil, stirring conatantly.
After it reaches a rolling boil, continue to boil two more minutes on stove without stirring, until it reaches soft ball stage.
Remove from heat, add vanilla and peanut butter.
Beat until smooth, then quickly spread over completely cooled brownies. This will work in a 9x13 or 8x8 pan, you'll just have more or less frosting.
****Note: my frosting was a little on the thin side (I'm not sure that I boiled it long enough), so I let it cool in the freezer to thicken up a bit before frosting the brownies. Once it cooled it was perfect, but I was a little unsure to start!