Place the diced rhubarb in a small bowl and toss with 1 teaspoon sugar. Set aside while preparing the cookie dough.

In a large mixing bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy, about 2 to 3 minutes.

Add the egg, egg yolk, vanilla extract, and cornstarch. Mix until smooth and well combined.

Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.

Blot the rhubarb with paper towels to remove excess moisture. Fold the rhubarb and chocolate chips into the dough.

Cover and refrigerate the dough for at least 1 hour.

Preheat the oven to 350°F. Line baking sheets with parchment paper.

Scoop large portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 11 to 13 minutes, or until the edges are set and the centers still look slightly underbaked.

Allow the cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.