How to Make It
Preheat the oven to 350°. Line an 8x8 inch pan with parchment paper, leaving a little overhang on 2 of the edges.
Spread the batter evenly into the prepared pan. Bake on the middle rack in the oven for 22 minutes. Remove from the oven and let cool completely.
When the shortbread is cool begin the caramel. Add all the caramel ingredients to a small but deep saucepan. Over medium heat stir until the butter has melted. Attach a candy thermometer and let the caramel boil (do not stir) until the temperature reaches 250°, about 9 minutes. Immediately remove the pan from the heat and pour the caramel over the cooled shortbread. Let the caramel harden at room temperature for about 10 minutes.
Using the overhanging parchment paper, remove the shortbread from the pan. Cut into 12 equal bars.
In a small pan over medium heat combine the dark chocolate, butter and 2 tablespoons of water. Stir until the chocolate has just melted then immediately remove from the heat. If the chocolate is quite thick add water, 1 teaspoon at a time, until it pours easily off a spoon. Drizzle the chocolate over top of each of the bars.