In a small skillet, melt 2 tablespoons of butter over medium heat.
Add chopped pecans and toast, stirring frequently, for 4-5 minutes until fragrant and golden brown.
Remove from heat and let cool completely.
In a medium bowl, whisk together the flour, cornstarch, salt, baking powder, and baking soda. Set aside.
In the bowl of a stand mixer, cream the cold butter, granulated sugar, and brown sugar on high speed for 3-4 minutes until light and fluffy.
Add the egg and egg yolk, one at a time, beating after each addition. Mix in the vanilla extract.
Reduce the mixer speed to low and add the dry ingredients in 3 parts, mixing until just combined.
Fold in the buttered pecans and chocolate chips.
Scoop dough into 1-tablespoon-sized balls and place on a parchment-lined baking sheet.
Refrigerate for at least 1-2 hours.
When ready to bake, preheat oven to 350°F. Bake cookies for 10-13 minutes, or until edges are golden and centers are just set.
Let cool on the pan for 4-5 minutes before transferring to a wire rack.