In medium saucepan over medium-low heat, combine brown sugar, butter, heavy cream, and corn syrup.
Stir until butter is completely melted.
Increase heat to medium-high and, stirring constantly, bring the mixture to a boil.
Boil for two minutes, stirring constantly, and then remove from heat. Continue to stir frequently so caramel doesn't burn.
Allow to cool 5 minutes then stir in vanilla extract.
Allow caramel to cool completely before proceeding (otherwise the heat from the caramel may melt the sugar and you will just have a mess on your hands).
Once caramel has cooled completely, transfer to KitchenAid and, with mixer on low-speed, gradually stir in sifted powdered sugar and corn starch (if mixture is too sticky you may need to add up to another ½ cup sugar).
Cover and refrigerate for 2-3 hours (or overnight).
Roll mixture into 1" balls and place on wax-paper lined cookie sheet (if the mixture begins to be too warm or sticky, place in the freezer for 10 minutes before resuming).
Place in freezer for 10-15 minutes.
Prepare chocolate by melting in large bowl for 30 seconds, stirring, and then repeating at 15 second intervals until chocolate is completely melted.
Dip buttercreams in chocolate***, transfer to wax-paper lined surface and allow the shells to harden before serving.
Sprinkle sparingly with sea salt.
Recommended that they be kept refrigerated
Notes
!*I prefer to use Ghirardelli dark chocolate melting wafers--they go on much cleaner and set much faster than regular chocolate chips. !**Add the sea salt very sparingly, even more-so than pictured in these photos. A little goes a long way. !***Prepare chocolate by melting in large bowl for 30 seconds, stirring, and then repeating at 15 second intervals until chocolate is completely melted. Dip buttercreams in chocolate***, transfer to wax-paper lined surface and allow the shells to harden before serving.