In a medium bowl, whisk together the dry ingredients: gluten-free flour, cornstarch, baking powder, and baking soda. Set aside.

In a stand mixer, beat the unsalted butter, light brown sugar, granulated sugar, and kosher salt on medium speed until light and fluffy.

Add the egg and egg yolk one at a time, mixing well, then add the vanilla extract.

Gradually blend in the dry ingredient mixture until just combined, then fold in the mixed chocolate and toasted pecans.

Scoop the dough into 6oz portions, place on a lined baking sheet, cover with plastic wrap, and refrigerate for at least 12 hours.

Preheat oven to 375°F (190°C), line baking sheets, space out the chilled dough, and flatten slightly.

Bake in preheated oven for 10 minutes, then lower temperature to 350°F (175°C) and bake for an additional 6-9 minutes.

Let cookies cool on baking sheets for 5-10 minutes before transferring to a wire rack.