Preheat the oven to 375 degrees. Line two baking sheets with parchment paper and set aside. Remove the butter from the fridge and cut into cubes. Place it in the bowl of a stand mixer fitted with the paddle attachment.
In a microwave safe bowl, melt 4 ounces of chopped chocolate or chocolate chips in 30 second increments. Stir between each cycle until smooth and shiny. Set aside to cool for 5 minutes while you make the dough.
Cream the butter with the granulated and light brown sugars for 3 minutes until lighter in color. Scrape down the sides of the bowl.
Add the egg and egg yolk, one at a time until fully incorporated. Add the vanilla and mix to combine. Scrape down the sides of the bowl.
In a medium bowl, combine the 2 cups (250 g) of flour, ¾ cup (95 g) of bread flour, baking soda, cornstarch and salt. Whisk to combine. Add the dry ingredients to the sugar and butter mixture and mix until just combined. Do not over mix.
Divide the dough in half. It’s helpful to use a kitchen scale to ensure the dough is divided in equal portions. Place one half back in the bowl of the stand mixer. Add the additional tablespoon of flour and mix until just combined. This will be your plain dough. Add the chocolate chips to the dough now if desired. Place the other half of the dough in a medium bowl and add the melted chocolate and cocoa powder. Stir with a spatula or wooden spoon until well combined. This will be your chocolate dough.
Use a kitchen scale to divide each of the doughs into .5 ounce portions. If the dough is tacky, dampen your hands or spray lightly with non-stick baking spray before rolling into balls. It is helpful to roll all of the vanilla dough balls and then wash your hands before rolling all of the chocolate balls. Once all of the cookie dough is portioned out, take two plain dough balls and two chocolate dough balls. Stack them together and then roll them between the palms of your hands to marble the dough together. Divide the dough in half with your fingers and turn one dough ball 90 degrees. Bring the two portions back together and smooth into a ball with your palms once more. The more you repeat this step, the more marbling your cookies will have once baked. Do not overdue it as the dough will lose the marbling effect.
Bake the cookies for 5 to 6 minutes and then remove the pan from the oven and tap it firmly on the countertop twice to help flatten the cookies. Rotate the pan and return it to the oven for 3 to 4 minutes more. The cookies are done when the edges are set and the centers are puffy. Tap the baking sheet once more on the countertop.The cookies will deflate slightly as they cool. Remove the pan from the oven and let cool for 5 minutes before transferring to a wire rack to cool completely.