Beat the butter with the sugars until creamy, using a stand mixer or hand-held beaters on medium speed for 2 minutes. It should look like caramel.
Add vanilla and egg, then beat again on low speed for about 10 seconds.
Add flour, baking soda, and salt. Mix only a couple of rotations until crumbles form, about 30 seconds.
Add 3 cups of chopped chocolate candy. Mix for 30 seconds.
Grease a 9×13 inch pan and spread the dough into one even layer. (It’ll rise as it cooks so don’t worry if it seems thin.)
Bake for 12 minutes. Remove the pan from the oven and stud the top with a handful of reserved candies. Cook 15 additional minutes until the edges are golden.