Add in 1 cup of butter, egg and vanilla extract and mix until everything is well combined.
Press out the crust into a 9 x 9 baking dish lined with parchment paper, prick the bottom with a fork and bake for 20 minutes or until slightly golden and cooked through.
I made the caramel sauce as the crust was cooking, and it was ready just a few minutes after I pulled the crust out of the oven. Allow the crust to cool slightly, then pour the caramel sauce over and spread it out evenly.
Let it sit at room temperature for 10 minutes, then chill in the fridge for at least 30 minutes and allow it to set before moving onto the chocolate layer.
Pour the chocolate chips and 2 tablespoons of butter into a heat proof mixing bowl set over a double boiler and stir just until the chocolate has melted.
Pour the chocolate over the caramel sauce and smooth using an offset spatula to get an even layer. Sprinkle some flaked sea salt over the top and chill in the fridge for several hours or overnight before cutting into small squares.
This is where you have to be patient, because if you cut into it too soon, the caramel will ooze out and make a mess. Store leftovers in an airtight container in the fridge for up to 5 days.
These are truly very rich little bars, living up to their name Millionaire's Shortbread. Be sure to cut them into 12 bars, perhaps even 16 because they are very sweet.