Whisk together baking soda, cornstarch, flour and salt in a large bowl.
In a separate bowl, whisk together brown sugar, butter and granulated sugar until a smooth paste. Add in egg and egg yolk and whisk until combined. Finally, add in the vanilla.
Pour the wet ingredients into the dry and gently fold in using a spoon or rubber spatula. Add in chocolate chips and refrigerate for 3 hours to 3 days. This is critical!
Preheat oven to 325 and remove dough from fridge. Let soften for 10 minutes before scooping balls of dough, 1 tablespoon in size, onto parchment lined baking sheets. Place only 8 balls on each sheet and use your fingers to press dough down slightly to flatten them.
Bake cookies for 12 minutes. Remove from oven and immediately press down once more with a spatula. Let cool for 10 minutes on the baking tray before transferring to cool completely on a wire rack.
Once completely cool, line half of the cookies, bottom-up on a tray next to your caramel station. Prepare an 8×8 baking pan by lining with parchment and spraying with cooking spray.
Melt butter and salt into the heavy cream over medium heat and set aside for later.
In a 4-quart saucepan, mix together corn syrup, sugar and water until it forms a grainy paste. Brush down the sides of the pan with a wet pastry brush, removing any extra sugar crystals. Clip on your candy thermometer. Do not stir after this point!
Place sugar mix over medium-high heat and bring to a boil. Continue to let the caramel boil without stirring, until the thermometer reads 320 degrees. Immediately remove from heat and pour in the cream mixture while gently whisking the sugar mixture. Stop whisking once all the cream has been added.
Return the pan to medium-high heat and heat the caramel to 245 degrees. It will turn from a lighter yellow to a darker caramel color and double in size. Remove from heat when it reaches 245 degrees and quickly whisk in the vanilla.
Immediately pour caramel into the prepared pan. Use a flat or offset spatula to scoop and spread caramel onto the bottom cookies. Sandwich the bottom cookies with the remaining cookies.
Melt semi-sweet chocolate in microwave in 15 second intervals until fully melted. Drizzle tops of cookies with chocolate. Allow chocolate and caramel to fully set before serving.
I didn’t have any unsalted butter, so feel free to use salted butter, just don’t add the additional salt in the recipe!
I used golden corn syrup, but light corn syrup would work as well.