Whisk together the flour and salt in a small mixing bowl, then set aside.

Cream the butter and sugar in the bowl of an electric stand mixer fitted with the paddle attachment until light and fluffy, stopping a few times to scrape down the sides and bottom of the bowl. Then, add in the vanilla and egg yolks and mix, stopping once to scrape the sides and bottom of the bowl, until everything is thoroughly mixed.

With the mixer on low speed, slowly and gradually add in the flour/salt mixture and mix just until everything is thoroughly combined. Avoid over-mixing, and you may have to stop and finish mixing with a wooden spoon to get all the dry ingredients from the bottom incorporated thoroughly.

Line 2 large baking sheets with parchment paper. Then, scoop cookie dough out by Tbsp. (I actually use the small Oxo Good Grips Cookie Scoop found here, which holds 2 tsp. of dough, and I scoop slightly heaping scoops so it ends up being ~1 Tbsp.), roll into ball shapes and place on the parchment-lined baking sheets. Once on the baking sheet, use the back of a small measuring spoon or your thumb to make indentations in the center of each cookie (keep in mind, the wider and deeper the indentation, the more caramel will fit in!). NOTE: I like to get all the cookie dough balls onto the baking sheets first, then make the thumbprint indentations in all of them. It seems to go faster this way.*

Place the baking sheets with the unbaked cookies in the refrigerator for ~60 minutes, until they have chilled and are no longer soft to the touch. After they're chilled, preheat oven to 350 degrees F and pull one of the baking sheets out of the refrigerator.

Once the oven is preheated, bake the cookies for ~15 minutes, until set. When the cookies are done, remove them from the oven immediately and, using the back of a small measuring spoon, press down on the indentations in the center of each cookie, as they will have flattened out during baking. Again, the deeper the indentation the more caramel you can fit in it!

Allow the cookies to cool on the baking sheet for 5 minutes, then transfer to wire cooling racks to cool completely. While the cookies are cooling, make the caramel sauce.

Begin by gathering everything, and I do mean everything, that you will need and place it next to the stove. This includes all of your ingredients (with butter already in pieces) as well as utensils. I usually have a couple of heat-resistant silicone spatulas and a pastry brush sitting in water for use, if needed, during cooking.

Place the sugar in a medium-sized nonstick saucepan over medium heat (very important that the heat be no higher than medium) and heat, stirring constantly with a heat-resistant spatula, for ~12 minutes until sugar completely breaks down into a dark amber-colored liquid and no longer looks grainy, being careful not to let it burn (seriously, stir nonstop. If you don't, the sugar will burn and it will be ruined).

---

Note #1: You'll feel like you're stirring forever with nothing happening but around the 8 minute mark it'll start forming some clumps and by 10 minutes it should be almost all clumps. Everything will go very fast from there on out.

---

Note #2: When the mixture is all liquid, toss the spatula you've been using into the sink (it will probably have some hardened sugar on it) and quickly wipe the inside of the pan down with a wet pastry brush or wet paper towel to remove any sugar crystals that are stuck to the side (DO NOT scrape sides with spatula). If some sugar chunks from your spatula got into the liquid, keep stirring (using a new heat-resistant spatula) and they should dissolve.

Moving very quickly, add the butter to the pan and, using your fresh heat-resistant spatula, stir nonstop and vigorously (but carefully to avoid splatter as the mixture is VERY HOT) for ~2-3 minutes until the butter is completely mixed in (butter will be separated for a while, but vigorous stirring will get it to thoroughly incorporate).

As soon as the butter is thoroughly incorporated and your mixture is nice and thick, add the cream SLOWLY with one hand (the cream is colder than the caramel at this point and everything will splatter like crazy if added too quickly) while stirring quickly & constantly with the other, until everything is nice and smooth and thoroughly mixed (about a minute).

As soon as the cream is mixed in and the caramel is nice and smooth, STOP STIRRING (very important) and allow to boil for 1+1/2 minutes. Remove from heat and wipe the inside of the pan again with a wet pastry brush or wet paper towel to get any remaining crystallized sugar off.

Allow the pan to sit just until the mixture stops boiling, then immediately pour the caramel sauce into a heat-resistant container (DO NOT scrape edges of pan at all, just pour and let whatever is stuck on the pan stay there. Trust me, you don't want it). Once in the heat-resistant container, allow the sauce to sit at room temperature for 3 minutes, then add in the vanilla extract and stir in gently until thoroughly incorporated. Note: You don't want to add the vanilla in right away or else it will all boil off and also may cause the caramel to crystalize a bit. Waiting 3 minutes lets the caramel cool just enough that it's safe to add the vanilla.

After the vanilla is mixed in, allow the caramel sauce to continue to cool at room temperature for ~5 more minutes before using. You don't want it to fully thicken or it will be hard to work with, but it definitely needs to have cooled down before use. You'll probably have more than you need for this recipe. I recommend eating the leftovers straight from the jar!

Once the caramel has cooled enough to use, fill the cookies with it. I do this by using a 1 tsp. measuring spoon to scoop caramel and a silicone spatula to scrape the bottom so it doesn't drip, and then pouring it in the center of each cookie. This could be a bit messy if you've never worked with caramel sauce before, though. You could use a ziploc bag with an end of one corner cut off or piping bag instead if you feel more comfortable, just make sure the caramel has cooled enough so that it won't melt the bag and won't come out so fast that you can't control it. Once all the cookies are filled with caramel, allow them to sit until the caramel has completely cooled before drizzling with the chocolate drizzle.

When the caramel in the cookie centers has fully cooled/firmed up, make the chocolate drizzle. Melt the chocolate chips with the shortening in a small dish in the microwave in 10 second spurts, stirring between each, until completely melted and smooth. Using either a piping bag or a ziploc bag with a small end of one corner cut off (cut off the end after you put the chocolate in), drizzle the chocolate over the cookies.