Brown the butter in a saucepan over medium heat until it smells nutty and turns amber. Cool 10 minutes.
Heat oven to 350°F. Line two baking sheets with parchment.
Toast the chopped pecans in a dry skillet over medium heat until fragrant, 3–4 minutes. Let cool.
Whisk flour, cornstarch, baking soda, salt, and cinnamon in a bowl.
Stir browned butter, brown sugar, and granulated sugar until smooth and glossy.
Whisk in the egg and vanilla until fully combined.
Fold the dry ingredients into the butter mixture just until a soft dough forms.
Stir in the toasted pecans. If the dough feels warm, chill 10 minutes.
Scoop 1½-tablespoon portions. Roll in coarse sugar if using. Space 2 inches apart on sheets.
Bake 10–12 minutes until edges set and centers look slightly soft. Cool 5 minutes, then move to a rack.