Melt butter and brown sugar together on low heat. Bring to boil and boil 2 minutes, stirring constantly. Add milk and heat just until boiling again. Remove from heat and cool slightly. Mix in powdered sugar until smooth. (I find using a whisk really helps in getting it smooth.)
Several things to note: The icing will be pretty runny but it WILL set up firm. This makes plenty for a 9×13 cake- in fact you might not even want to use it all if you aren’t much of a sweet tooth.
Recipe by The Thrifty Frugal Mom at http://www.parents.com/blogs/thrifty-frugal-mom/2013/11/12/recipes/tasty-tuesday-quick-caramel-icing/