Prep the Granny Smith Apples by inserting the sticks and removing the wax. (see post for more details) Set aside to dry completely.
In a medium non-stick sauce pan, combine the butter, brown sugar, sweetened condensed milk and corn syrup.
Bring to a rolling boil, stirring constantly. Boil until the caramel has reached the almost hardball stage. (235°-240°). If you don't have a candy thermometer, see post for details on how to check the stage of the caramel.
Remove from heat and stir in the marshmallows.
Gently fold the caramel to remove any built in air bubbles.
Dip the apples into the caramel. See post for tips on how to best execute this.
Pour any excess caramel onto parchment paper and then cut into caramels when cool.
Once the caramel apples have cooled completely, add toppings like chocolate drizzle, nuts, sprinkles or anything else you'd like!