Melt butter. Stir in brown sugar and salt. Boil for 2 minutes, stirring constantly. Remove from heat, add milk and return to heat. Bring to rolling boil. Cool to lukewarm and stir in powdered sugar. Add a bit more if necessary for piping. Fill a pasty bag with a #2A tip (or #230 bismarck tip for mini cupcakes) with the caramel filling.
Recipe and image taken with permission fromVi Whittington and Autumn Carpenter's Book, Creative Cupcakes.