Step 1: Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Step 2: In a large mixing bowl, beat together the softened butter, brown sugar, and granulated sugar using an electric mixer until light and fluffy, about 2-3 minutes.
Step 3: Add the egg and vanilla extract to the creamed mixture. Mix until fully combined, scraping down the sides of the bowl as needed.
Step 4: In a separate bowl, whisk together the flour, cornstarch, baking soda, and optional salt.
Step 5: Gradually add the dry mixture to the wet ingredients, mixing until just combined.
Step 6: Gently fold in the diced Twix bars and chocolate chips using a spatula.
Step 7: Drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are lightly golden.
Step 8: Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.