Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly and develop that perfect golden color.
In a large mixing bowl, use an electric mixer to cream together the softened unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Add in the large egg and vanilla extract to the creamed mixture. Beat until fully combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the all-purpose flour, bread flour (or additional all-purpose flour), cornstarch, baking soda, and salt to evenly distribute the leavening agents.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Avoid overmixing to prevent tough cookies.
Using a spatula, gently fold in the semi-sweet chocolate chips and chopped bittersweet or dark chocolate until evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop heaping spoonfuls of dough onto prepared baking sheets, leaving space between each cookie to allow for spreading.
Bake in the preheated oven for 10-12 minutes, or until edges are lightly golden but centers remain soft for a chewy texture.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Enjoy warm!