• Preheat oven to 350F (177C) and line a 13x9 baking pan with parchment paper.
• Using an electric mixer, beat butter and sugar until light, fluffy, and well-creamed.
  1 cup unsalted butter, ⅓ cup granulated sugar, ⅓ cup light brown sugar, packed
• Add egg yolk and vanilla extract and beat well, pausing to scrape down the sides and mixing until ingredients are well-combined.
  1 large egg yolk, ¾ teaspoon vanilla extract
• Gradually, with mixer on low-speed, add flour, pausing to scrape down sides of bowl. Halfway through the addition of the flour, sprinkle in the salt with the beater still running. Don't over-beat the dough, you want it to be slightly crumbly to make it easier to press into the pan.
  ½ teaspoon table salt, 2 ¼ cups all-purpose flour
• Distribute dough evenly into prepared pan and gently, firmly, evenly, press dough into the bottom of the pan. To make this easier, I usually lay a piece of wax paper over the dough and use the back of a spoon to smooth the surface (and then remove/discard wax paper).
• Transfer to center rack of 350F (175C) preheated oven and bake for 25 minutes, until edges are light golden brown. Allow to cool while you prepare your caramel topping.

For the Caramel Layer

• Combine condensed milk, butter, brown sugar, corn syrup, and salt in a medium-sized saucepan over medium-low heat.
  2 14-oz cans sweetened condensed milk, 14 Tablespoons unsalted butter, 1 cup light brown sugar, ⅓ cup light corn syrup, ¾ teaspoon table salt
• Stir frequently until butter is melted and ingredients are well-combined. Continue to stir constantly until mixture has thickened, turned caramel in color, and begins to bubble (do not turn up the heat too high or the mixture could separate). Temperature on an instant-read thermometer should reach 225F (107C). Note this could take 15 minutes or even longer -- depends on the specific heat of your stove!
• Remove caramel from heat and immediately stir in the vanilla extract.
  1 teaspoon vanilla extract
• Pour evenly over prepared shortbread.
• Allow to cool at room temperature for several hours or in the refrigerator for 1 hour before topping with chocolate.

Chocolate Topping

• Combine chocolate chips and heavy cream in a small saucepan over low heat.  
  ½ cup heavy cream, 2 cups semisweet chocolate chips
• Stir frequently until chocolate is melted and mixture is smooth.  
• Remove from heat and allow chocolate to cool just slightly for about 5 minutes, and then evenly spread over prepared caramel layer.  Wait several minutes/until chocolate begins to set and then sprinkle with sea salt.  
  flaky sea salt
• Allow chocolate to firm up (I recommend placing in refrigerator for about 30 minutes -- my family prefers these treats refrigerated anyway) before cutting and serving.