If using bar chocolate, chop your chocolate into chunks and set aside.
In a large pot, melt the butter over medium heat. It will start to crack and sizzle - let it do this while stirring occasionally. After a few minutes or so, the butter cracking/sizzling will subside and the butter will begin to foam up in the pan.
Every few seconds or so, swirl the pan around until you can see that the butter is golden brown. You should also be able to see some of the brown bits swirling around in the butter. Watch it carefully, you don't want it to burn. Once golden brown, immediately remove from the heat and pour the butter into a large bowl on a scale. You should have 170gms of brown butter or if you don't have a scale measure out 3/4 cup. Pour the measured amount into a bowl and put it in the fridge to cool for 10 minutes.
After 10 minutes, take the butter out of the fridge and add the brown sugar, granulated sugar, and vanilla extract. Whisk to combine.
Add the egg and whisk well until completely combined. Add the sour cream and blend.
Add the baking soda, baking powder, cornstarch, and salt, and whisk until incorporated.
Add the flour and fold it into the mixture using a rubber spatula until there are only a few streaks of flour remaining. Add the chopped chocolate and fold until evenly incorporated and the flour is fully mixed in.
Cover the bowl tightly and chill the dough for 3 hours. Once the three hours is almost up, preheat your oven to 350F and line a baking sheet with parchment paper.
Scoop the dough using a cookie scoop (use a 1.5 oz scoop #24, about ~3 tablespoons or ~50 grams) and space them 2-3 inches apart on your baking sheet.
Bake for 10-11 minutes until the edges are set and the centers still look slightly underdone. Rotate pan halfway through baking. Let cool on the baking sheet for 10 minutes, then enjoy warm or transfer to a baking sheet to cool completely. See post for notes on perfectly round cookies with ridges.