Preheat the oven to 350°F. Line 2 large baking sheets with parchment paper.
In a small or medium mixing bowl, whisk together the flour, cornstarch, baking soda, and salt. Set the bowl aside.
In a separate large bowl, use an electric mixer to mix the brown sugar, sugar, butter, and vanilla extract until fluffy.
Pour in the buttermilk and continue mixing.
Mix in the dry ingredients a little bit at a time until it has all been incorporated.
Fold in the chocolate chips until they are evenly distributed.
Use a medium (1-½ tablespoon) cookie scoop to scoop out level balls of dough and drop them on the parchment-lined baking sheets. Place the dough balls about 2 inches apart. I could fit 12 cookies on each baking sheet for a total of 24 cookies.
Press a few extra chocolate chips on top of each dough ball. Optional, but it adds more chocolatey-ness.
Bake the cookies for 15-17 minutes or until the cookies have spread slightly and are brown around the edges.
Let the cookies rest on the baking sheet for 15 minutes before transferring them to a cooling rack. Allow them to cool completely, then serve fresh or store in an airtight container for 5 days.