In a large mixing bowl, beat the butter with an electric mixer on medium speed until smooth. Beat in the brown sugar and powdered sugar until creamy, about 2 minutes. Add in the vanilla extract and do a quick mix to combine.

Add in the flour and salt and mix at low speed to combine. Before the flour is completed mixed in, add the chopped chocolate and finish mixing at low speed.

Turn the cookie dough out onto a sheet of plastic wrap. Press the dough together with your hands to form a cylindrical, log shape. Loosely fold over the plastic wrap and roll the log against the countertop with your hands until your log is about 12" in length and 2" in width (diameter). Cut the log in half for easier storage and wrap both logs in plastic wrap. Chill in the fridge for at least 1 hour or overnight until they are solid throughout. 

Bake for about 15 minutes or until the edges become golden brown. Sprinkle cookies with flaky salt while they're still warm. Let cookies cool completely before enjoying them as they need time to set.