Place the butter and brown sugar into the bowl of a stand mixer. Beat on low speed until combined. Make sure not to overbeat the butter.

Next, add the egg and vanilla extract to the butter/sugar mixture. Beat until just combined.

Add the flour and salt to the rest of the ingredients in the mixing bowl and mix on low speed until a dough forms. (The dough will crumbly.)

Finally, add the chopped chocolate and mix on low speed until it is evenly distributed throughout the cookie dough.

Transfer the dough onto a work surface and shape it into a log, about 2-2 1/2 inches in diameter. Tightly wrap the log in plastic wrap and chill in the refrigerator for at least 2 hours.

Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper. Unwrap the chilled log and slice it into 1/4-inch thick rounds. Place the sliced cookies onto the prepared baking sheets, about 1 inch apart. Bake the cookies for 10 minutes.

Remove the cookies from the oven and allow them to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. The cookies will harden as they cool.