Chop your chocolate and set aside.
100 grams semi-sweet chocolate, 100 grams dark chocolate
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.
211 grams unsalted butter (European style is preferred!)
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.
Once golden brown, immediately remove from the heat and pour the butter into a large bowl and put in the fridge to cool for 15-20 minutes.
Remove the butter from the fridge and add the brown sugar, granulated sugar, and vanilla extract. Whisk to combine.
150 grams light brown sugar, 50 grams granulated sugar, ½ tablespoon vanilla extract
Add the egg and sour cream and whisk to combine.
1 egg, 20 grams sour cream
Add the baking soda, baking powder, cornstarch, and salt, and whisk until incorporated.
1 teaspoon baking soda, 1 teaspoon baking powder, 1 teaspoon cornstarch, 1 teaspoon fine sea salt
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.
225 grams all purpose flour
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 (I used a 1.5 oz scoop, 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. Let cool on the baking sheet for 10 minutes, then enjoy warm or transfer to a baking sheet to cool completely.