- 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.