Preheat the oven to 350°F

Coat a 9x13-inch pan with oil spray and line the pan with parchment paper long enough to extend 3-inches on two ends. Spray the parchment paper with oil spray.

Add the flour and sugar to a food processor and pulse to combine. Add the melted butter and pulse until the flour is moistened and crumbly. 

Evenly spread the dough across the bottom of the pan, pressing it down until it's smooth and uniform in thickness. Afterward, use a fork to pierce the dough at 1-inch intervals.  

Bake the crust for25-30 minutes or until golden brown. Use a metal spatula to firmly down on the crust while it is still warm. This step ensures the bars will be easier to cut later on. Set aside to cool.

Add the condensed milk, sugar, cream, syrup, and butter to a heavy-bottomed saucepan set over medium heat. Cook, stirring frequently, until a candy thermometer or an instant-read thermometer reaches 236-239°F, about 20 minutes.

Remove from the heat and stir in the vanilla and salt.

Pour the salted caramel over the slightly warm crust and spread evenly.  Allow to cool completely before adding the chocolate topping, about 2 hours.

Heat the cream in a small saucepan over low heat, do not allow it to boil.  Alternatively, place in a microwaveable dish and zap it in increments of 15 seconds until very hot.

Place the chopped chocolate in a bowl and pour the hot cream over the top. Cover the bowl and let it sit for 5 minutes, then stir. If it isn't completely melted, finish melting the chocolate in the microwave with short 5 second bursts, (stirring between bursts).

Pour the chocolate over the salted caramel filling, spreading it evenly. Let sit for 5 minutes, then sprinkle the surface with large salt flakes. 

Transfer the pan to the refrigerator and allow it to cool, about 15-20 minutes. 

Using a thin, sharp knife slide along the long sides of the pan to loosen the shortbread. Gently, lift the shortbread out of the pan, using the parchment paper ends. 

Transfer the shortbread to a cutting board. Slice it into bars using a serrated knife, dip it in warm water, and wipe it dry before each cut.