Place butter in a medium saucepan (with a silver bottom, not dark/teflon or you won't see the color change) over medium heat. Whisk butter as it melts. Continue to whisk as the melted butter becomes almost foamy. Keep whisking, and don't walk away from the pan. The butter foam will eventually settle and the butter will continue to bubble. Keep whisking! Eventually you'll notice the butter will start to brown and there will be a nutty aroma. Once butter is browned and smells nutty, immediately remove from heat and pour into another bowl. This process happens fast and you don't want it to burn!

Take bowl of butter and set it in the fridge to cool down while you get out and prepare the rest of the ingredients. The butter should cool until it's warm (or room temperature) but certainly not hot.

In a medium bowl, whisk together flour, baking soda, cornstarch, and salt. Set aside.

Combine both sugars and browned butter in mixer bowl with whisk attachment. (The butter should still be melted, but not super hot.) Whisk on high for 20 seconds. Scrape down walls with a spatula, then add room temperature egg, additional yolk, and vanilla. Whisk on high for 30 seconds. Scrape down sides. Add Starbucks VIA or coffee powder and whisk on high again. The mixture should now be thick, glossy, and completely combined (with small coffee specks).

Switch mixer from whisk attachment to paddle attachment. Slowly add in flour mixture, about half a cup at a time. Use spatula to scrape sides and bottom of mixer bowl when necessary. Mix until flour mixture is just combined, but do not over mix. Add in chocolate chunks and then chill dough for at least two hours in the refrigerator (I prefer overnight for convenience). 

Preheat oven to 350 degrees. Scoop dough into 1 1/2" balls and bake on parchment paper for 12-14 minutes. Top each cookie with a pinch of flaky salt immediately after removing from oven.