Preheat your oven to 350°F (175°C).
Cream together the softened butter, light brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Add the egg and vanilla extract, mixing until well combined.
In another bowl, whisk together the flour, cornstarch, baking soda, and salt.
Gradually add the dry mixture to the wet ingredients, mixing until just combined.
Fold in the diced Twix bars and chocolate chips until evenly distributed.
Scoop the dough onto a parchment-lined baking sheet, leaving about 2 inches between each cookie.
Bake for 10-12 minutes or until the edges are lightly golden.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.
