Heat a skillet over medium heat. Add butter and allow to melt. Whisk butter gently, almost constantly. Continue cooking the butter. It will start to foam. Watch the bottom of the skillet. Brown bits will start to form. Once they are dark golden, the butter is done. Pour into a heat safe container and refrigerate for about 45 minutes, until it's solidified but spreadable.
Add butter to the bowl of a stand mixer. Add brown and granulated sugar. Cream for 5 minutes at medium speed, until light and very fluffy. Add eggs, one at a time, mixing well between additions. Add vanilla extract, mix.
In a large bowl, combine flour, baking soda, salt and cornstarch. Add half the dry ingredients and mix until just combined. Add remaining dry ingredients and mix again until it just comes together. Stir in dark chocolate chunks.
Line a large baking sheet with parchment. Using a cookie scoop, portion out cookie dough onto the baking sheet. The dough will be refrigerated next so they don't need to be spaced out. Press down gently with two fingers. Refrigerate for 2 hours, or up to overnight.
When ready to bake, preheat oven to 350 degrees. Place on parchment lined baking sheets, 2 inches apart. Bake for 15 to 18 minutes, until light golden brown on the edges.