Start by preheating your oven to 350°F (175°C). This ensures that when you place your cookie dough into the oven, it’ll bake evenly and thoroughly.

Line a sheet pan (typically 18x13 inches) with parchment paper. This prevents sticking and makes for easy cleanup. Make sure to leave some overhang so you can lift the cookie out effortlessly once it’s done.

In a large bowl, whisk together the flour, cornstarch, baking soda, baking powder, sea salt, and cinnamon. This step is crucial. Mixing ensures that the leavening agents are evenly distributed, which is key for consistent texture throughout your cookie.

In another bowl, beat together the melted butter, dark brown sugar, and granulated sugar. Creaming these ingredients together activates the sugars and promotes a fluffy texture. You want this mixture to be smooth and well combined. Add in the eggs and egg yolks, mixing until fully incorporated.

Slowly add the dry mixture to the wet mixture, combining until just blended. Be careful not to over-mix, or you may end up with tougher cookies.

Now, fold in the chocolate chips and chocolate chunks. Feel free to sample a few—you know, just to make sure they’re good! This is the fun part—seeing all that chocolate goodness in the future cookie surface.

Spread the batter evenly across the prepared baking sheet. Use a spatula to smooth the top and make sure it reaches the corners of the pan.

Place the sheet pan in the oven for about 18 to 20 minutes, or until the edges are slightly golden and the center looks set. It's essential to keep an eye on it, especially toward the end of the baking time—nobody likes overbaked cookies!

Allow the cookie to cool in the pan for at least 10 minutes. This will help firm it up. Once cooled, use a sharp knife to slice it into squares or rectangles.