Combine the flour, cornstarch, baking soda, and salt in a large bowl. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine butter, brown sugar, and granulated sugar. Beat on high for 3 minutes.
Beat in egg and egg yolk, then beat in vanilla. Slowly add the dry ingredients to the bowl and mix on low speed until just combined. Be sure to not over mix here.
Remove the bowl from the stand mixer. Use a rubber spatula or a wooden spoon to mix in the chocolate chips and chopped caramel pieces until fully combined.
Cover the bowl in plastic wrap and let chill in the refrigerator for at least 2 hours or up to 3 days.
Preheat the oven to 350°F. Line 2 baking sheets with parchment paper. Scoop the cookie dough into balls and place on prepared pans (I have a 2 oz scoop that I use to portion mine). You may end up needing a third pan, but my oven only has 2 racks so I can only bake 2 pans at once.
Bake the cookies for 8 minutes. Rotate the pans 180° and put the bottom pan on the top, and the top pan on the bottom. Bake an additional 3 to 4 minutes or until the edges of the cookies are just beginning to turn golden brown.
Remove the cookies from the oven and immediately sprinkle them with flaky sea salt (doing this immediately will allow the salt to stick to the tops better), and let cool completely on the pans.
Store the cookies at room temperature for up to 3 days.