Stir frequently. Once sugar dissolves, increase heat and bring to a boil, then reduce back to medium-low heat. Cook, stirring frequently, for 8-10 minutes until mixture becomes thick and begins to pull away from sides of pan. Pour over cooked shortbread base and place in refrigerator to cool and firm. Approximately 15-30 minutes.
4. Once firm, melt chocolate in a heat-proof bowl over a saucepan of water (double-boiler). Heat chocolate, stirring frequently until melted. Cool slightly. Carefully pour over caramel. Chill, in fridge until set. Cut into bars. Store in an airtight container up to a week.