Preheat oven to 325°F. Line a 9x9-inch pan with parchment paper or foil and spray with nonstick cooking spray.

In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl with an electric hand mixer, beat the butter and granulated sugar together on medium speed until light and fluffy, about 3 minutes, scraping down the sides of the bowl as needed. Add the salt and flour and mix on low speed until incorporated and the mixture forms crumbs.

Press the shortbread mixture firmly into the bottom of the prepared pan. Bake until light golden, about 35-40 minutes. Transfer the pan to a wire rack and allow the base to cool while you make the caramel.

Add the sweetened condensed milk, butter, brown sugar and corn syrup to a medium saucepan. Place over medium heat and whisk until melted and smooth. Add a candy thermometer to the pot, then continue stirring continuously over medium heat until it bubbles and reaches 225°F. This will take about 5 minutes from when the mixture starts to boil. Remove from the heat and add the salt and vanilla; carefully stir to combine (the mixture will spit and bubble a bit).

Pour caramel over the shortbread base. Place the pan in the refrigerator to chill for about 30 minutes before adding the chocolate layer.

Place chocolate chips and heavy cream in a large measuring cup or medium sized bowl. Heat in 30 second increments until melted and smooth. Spread over caramel in a smooth layer. Chill for 2-3 hours, or until set.

Slice into bars. Store in an airtight container in the refrigerator for up to 4 days.