Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, baking soda, salt, cornstarch, and cinnamon. Set aside.
In a large bowl, cream together the butter, dark brown sugar, and granulated sugar until light and fluffy, about 3 to 4 minutes.
Add the eggs and egg yolk, one at a time, mixing well after each addition. Stir in the vanilla extract.
Gradually add the flour mixture to the butter mixture, mixing until just combined.
Fold in the coarsely chopped pecans.
Using a cookie scoop or a spoon, drop dough by rounded tablespoonfuls onto the prepared baking sheets, spacing them about 2 inches apart.
Gently press a pecan half onto the top of each cookie.
Bake the cookies for 12 to 15 minutes, or until lightly golden brown around the edges.
Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Once the cookies have cooled completely, store them in an airtight container at room temperature for up to 1 week.
Enjoy your delicious butter pecan cookies!
