1. Make the caramel sauce: Heat sugar in a medium saucepan over medium-high heat, stirring occasionally with a spatula or wooden spoon. Sugar will form clumps and eventually melt into a thick brown, amber-colored liquid as you continue to stir. Be careful not to burn. Once sugar is completely melted and reached a deep amber color, remove the saucepan from heat. Immediately pour heavy cream (Be careful, as the caramel will bubble up) and Whisk until all the cream has been incorporated and you have a smooth sauce. Return to heat and stir in butter and salt. stir until butter it is completely melted and the salt is incorporated. Set aside.
2. Place the chocolate in a double boiler or glass bowl over a simmering pot of water. Allow to melt completely, stirring occasionally.
3. Spoon 1-2 tablespoons melted chocolate into 8 baking cups, use a spoon to spread chocolate across bottom and up sides of each cup. Place the cups in the refrigerator for 10-15 minutes. Then spread another layer of chocolate and refrigerate for 10 minutes more.
4. Fill with 1 tablespoon of the caramel sauce. And top with remaining chocolate.
5. Sprinkle each chocolate with a little coarse salt and allow to set at room temperature.