Preheat oven to 350 and line a 9 x 13 pan with parchment.

For the Shortbread: In the bowl of a stand mixer, or with a medium mixing bowl and a hand mixer, blend butter, powdered sugar, flour and salt until incorporated and crumbly.  It should resemble wet sand.  Pour the mixture into the 9 x 13 pan and press the dough firmly and evenly along the bottom.  Bake for 18-20 minutes until the shortbread is set and golden.

For the Caramel:  Place the water, sugar, vanilla and salt in a heavy saucepan and heat over medium-high heat.  The sugar will begin to caramelize and gain an amber color.  As the edges darken, swirl the pan to even out the color and avoid burning the sugar.  The caramelization of the sugar will take 8-10 minutes depending on the level of your heat.  Once you have achieved the desired color add the butter and cream, being careful to avoid splashes and steam.  Place the candy thermometer in the caramel mixture and cook until the caramel has reached 245F or 118C.  This will take an additional 8-10 minutes.  When the caramel has reached temperature, remove from heat and stir as the boil slows.  Pour the hot caramel over the shortbread and allow to set for at least one hour before adding the chocolate layer.

For the Chocolate:  Once the caramel has set, place the chocolate chips and oil in a medium bowl and microwave to melt.  Microwave for 30 seconds, stir, and return to microwave for another 30 seconds.  Repeat as needed to melt the chocolate.  Pour the chocolate over the set caramel and sprinkle with (optional) flaky sea salt.