Chop your pecans 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. Once golden brown, immediately remove from the heat and pour the butter into a large bowl and put in the fridge to cool for 10 minutes.
After the 10 minutes, remove the butter from the fridge and add the brown sugar, granulated sugar, and vanilla. Whisk to combine.
Add the egg and whisk well until completely combined. Add the sour cream and whisk to combine.
Add the baking soda, baking powder, cornstarch, cinnamon, 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 pecans and 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 (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.