In a large mixing bowl, beat the softened butter, cream cheese, brown sugar, and granulated sugar with an electric mixer until light and fluffy (about 2-3 minutes).

Add the egg and vanilla extract, mixing until fully incorporated.

In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the chocolate chips using a spatula, ensuring they are evenly distributed throughout the dough.

Cover the dough and refrigerate for at least 2 hours (or overnight) to allow it to firm up and develop flavor.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

Using a cookie scoop or spoon, form the dough into 1.5-inch balls and place them on the prepared baking sheets, spacing them about 2 inches apart.

Bake for 9-11 minutes, or until the edges are set but the centers appear slightly underbaked. Do not overbake; the cookies will continue to firm up as they cool.

Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a cooling rack.