Preheat the oven to 350°F. Line a 9x13 (or 13x9) baking dish with parchment paper. Lightly spray the parchment with nonstick cooking spray.

Add the flour and kosher salt to a medium-sized mixing bowl (2-3 quarts). Whisk to combine.

Using a stand mixer or a medium-sized mixing bowl (2-3 quarts) and a handheld mixer on high speed, beat the softened butter for 1-1½ minutes until smooth.

Add the sugars and continue beating for another 1-1½ minutes or until well combined.

Lower the mixer speed to medium and add the egg yolk. Continue mixing just until no yellow streaks are visible. Add the vanilla and mix until no brown streaks remain.

Lower the mixer speed to medium-low. Add ½ of the flour mixture, mixing just until combined. Add the remaining flour and mix just until incorporated, do not overmix.

Press the shortbread crust into the bottom of the prepared baking dish. Be sure that the crust is evenly spread, or you will have a very lumpy base. Bake for 20 minutes or until the edges are light golden brown. Allow the crust to rest while you prepare the caramel layer.

Add the sliced butter pats, sweetened condensed milk, light brown sugar, corn syrup, and kosher salt to a 2 to 3-quart heavy-bottomed saucepan over medium heat. Stir continuously while the butter and sugar melt. The color will be a very light caramel color.

Continue stirring over medium heat until the caramel reaches 235°F on a candy thermometer and the caramel turns a golden color. Remove the saucepan from the heat and stir in the vanilla. Continue stirring until a uniform color is reached. Evenly spread the caramel over the shortbread crust. Use a silicone spatula or an offset spatula to evenly spread the caramel over the shortbread. Allow the caramel-covered shortbread to rest for 30-35 minutes before making the chocolate topping.

Add the heavy cream to a small microwave-safe bowl or a 1-cup glass measuring cup. Microwave for 40-45 seconds on high. Depending on your microwave, keep a close eye on the heavy cream. You want the cream to bubble but not bubble over. You can also use a small saucepan to heat the heavy cream until small bubbles appear and the cream is steaming.

Add the chocolate chips to a medium-sized heat-safe bowl (2-3 quarts). Pour the hot heavy cream over the chips. Allow the milk and chocolate chips to sit for 1-2 minutes to let the cream melt the chocolate. Allow the hot cream to do most of the heavy lifting for you. Stir the cream and chocolate mixture until the chocolate is completely melted and has a shiny appearance.

Top the caramel layer with chocolate ganache. Use a silicone spatula, or an offset spatula, to smooth the ganache. Garnish with sea salt, if desired. Chill in the refrigerator for 2 hours. Use a large sharp knife to slice 4 slices x 6 slices. Store in the refrigerator until ready to serve.