Begin by preheating your oven to 350°F (175°C).
In a large mixing bowl, beat together the softened unsalted butter, packed brown sugar, and granulated sugar until the mixture is light and fluffy, about 2-3 minutes.
Add the large egg and vanilla extract to the butter-sugar mixture and beat well until fully combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and cornstarch. Slowly incorporate these dry ingredients into the wet mixture, stirring until just combined.
Gently fold in the orange zest and chopped dark chocolate.
Using a cookie scoop or a tablespoon, drop rounded balls of dough onto a prepared cookie sheet.
Place the cookie sheet in the preheated oven and bake for 10-12 minutes, or until the edges are golden and the centers look slightly underbaked.
Remove the cookies from the oven and let them cool on the sheet for about 5 minutes before transferring them to a wire rack to cool completely.
