Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly.
In a large mixing bowl, cream together the softened unsalted butter, light brown sugar, and granulated sugar until the mixture is light and fluffy. This should take about 2-3 minutes with a hand mixer or stand mixer.
Beat in the large egg and the vanilla extract until fully incorporated.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and a pinch of salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined. Be careful not to overmix; you want the dough to be soft and slightly sticky.
Gently fold in the diced Twix and semi-sweet chocolate chips until evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a lined baking sheet, spacing them about 2 inches apart to allow for spreading.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden but the centers still look slightly underbaked. This is key for keeping them chewy!
Remove the cookies from the oven and let them cool on the baking sheet for about 5 minutes before transferring them to a cooling rack to cool completely. Enjoy the delicious aroma filling your kitchen!
