This cookie dough must be chilled before baking. For best results, chill rolled cookies for at least 3 hours (longer is better).
In the large bowl of a stand mixer fitted with a paddle attachment, beat1 cup butter, 1 cup granulated sugar, and 1 cup brown sugar together on medium high speed light and fluffy, about 2–3 minutes.
Add 2 eggs and 1 tablespoon vanilla, and then beat on high speed until combined. Scrape down the sides and up the bottom of the bowl as needed.
In a separate bowl, whisk together 2 cups flour, 1 1/3 cup cocoa powder, 2 teaspoons baking soda, and 1/4 teaspoon salt.
Turn the mixer to low and slowly pour the dry into the wet ingredients. Beat on low until combined. The cookie dough will be quite thick.
Increase to medium-high speed and beat in the 2 tablespoons of milk.
Reduce speed against and add salted caramel chips. The cookie dough will be sticky and tacky.
Using your hands, roll dough into tablespoon-sized balls and set on a parchment-lined tray 2-3 inches apart.
Cover pans with plastic wrap and chill in the refrigerator for at least 3 hours. Chilling is required for this sticky cookie dough.
Preheat oven to 350 degrees F.
Remove from refrigerator and lightly sprinkle sea salt on top of each, if desired.
Bake the cookies for 12–13 minutes or until the edges appear set. The centers will still look soft.
Cool cookies for 5 minutes on the baking sheet. Transfer to cooling rack to cool completely. The cookies will slightly deflate as they cool.
Cover leftover cookies tightly and store at room temperature for up to 1 week.