Preheat oven to 375°F. Line a baking sheet with parchment paper.
In a microwave-safe bowl, melt butter.
In a large bowl, add brown sugar. Pour butter over brown sugar and mix to combine.
Add in egg and vanilla and whisk until fully incorporated.
Add flour, baking soda, cornstarch, and salt to the brown sugar mixture. Mix by hand until combined. The batter will be coarse and appear dry.
Coarsely chop the chocolate and add to the batter. Reserve a few pieces of chocolate to add to the top of the cookies.
Using a cookie scoop that holds about 2 tablespoons of batter, scoop out the dough and place it onto the lined baking sheet.
Place a few (3-5) extra chunks on chocolate on top of each cookie.
Using a clean kitchen glass, press down each cookie until it is about ½ to ¾ inch thick. This step is optional, but if you don't press the cookies down they may need to bake longer as they do not flatten on their own.
Bake for 7-9 minutes or until the edges start to turn golden brown.
Remove from oven and let rest for about 5 minutes. Move to a cooling rack.