Preheat the oven to 350°F and fully line a 9x9 inch baking dish with parchment paper or foil.

In the bowl of a stand mixer with the paddle attachment, or using a hand mixer, beat the butter, sugar, and salt at medium-high speed until light and fluffy, about 3 minutes. Add the vanilla and egg yolk, then mix until combined.

While mixing on low speed, add the flour in a few batches and mix until just combined. Stop occasionally and scrape down the bowl to make sure everything is thoroughly mixed together. Transfer the dough to your prepared pan and press into an even layer. (Make sure there are no holes or gaps in the shortbread dough, or it can fall apart later on in the assembly process.)

Bake for about 22 minutes or until the edges are a light golden brown.

While the shortbread base bakes, make the caramel. Combine the sweetened condensed milk, butter, brown sugar, corn syrup, and salt in a small pot.

Place over medium heat and whisk occasionally until the butter is melted and combined, about 3 minutes. Then, begin whisking constantly until the caramel bubbles and darkens slightly, and reaches 225°F on an instant read thermometer. This will take 6-10 minutes. Once it reaches temperature, you'll see it start to thicken and pull away from the sides of your pan. Remove the pot from the heat and whisk in the vanilla.

When the shortbread is done, pour the hot caramel onto the hot shortbread base, and smooth it to the edges creating a flat layer. Set aside to cool to room temperature so the caramel can set on top.

Combine the chopped chocolate and heavy cream in a medium microwave-safe bowl. Microwave on high in 20-second bursts, stirring in between, until the chocolate is fully melted and the ganache is smooth, about 60 seconds. Pour the chocolate over the cooled caramel and smooth into an even layer. Sprinkle with salt if desired, then chill the pan for at least 2 hours.

To cut into pieces, use the parchment paper to lift the slab out of the pan. Warm a knife by running it under hot water, then dry it off. Slice the slab into bars with the warm knife (rewarm and wipe it clean as often as needed.)