Preheat the oven to 350°F. Spray a standard half sheet pan with nonstick spray (13×18) and set aside.
Add the melted butter and sugars together in a large mixing bowl.
1 1/4 cup unsalted butter, 1 1/4 cup brown sugar, dark or light, 1 cup granulated sugar
Whisk together until smooth. Be sure to break up any clumps of brown sugar.
Add the eggs, milk and vanilla. Whisk until smooth.
2 large eggs, 1 large egg yolk, 2 Tbsp. whole milk, 1 1/2 Tbsp. vanilla extract
Add the dry ingredients.
Fold until mostly combined. Do not over mix the cookie dough in this step.
3 cups all purpose flour, 1 1/2 tsp. kosher salt, 1 tsp. cornstarch, 1 tsp. baking soda, 1 tsp. baking powder
Divide the base cookie dough batter into 4 equal sections and place in 4 medium sized bowls. You can use a food scale to weigh each portion of so they are more accurate if you'd like, each quadrant will be ~10-11 oz. each.
Add additional mix-ins to each bowl and mix just until combined.
Place each cookie portion onto a quadrant of your sheet pan.
Using a rubber spatula, flatten each section down evenly. Do the best you can, it doesn't have to be perfect. The cookie dough should fill the entire sheet pan and they cookie doughs will be touching.
Bake for ~18-20 minutes, do not over bake. Slightly underbake because there will be carry over cooking as it cools on sheet pan. It will look puffy at first, but settles as it cools. You can gently tap it on the counter if it’s really fluffy so it settles, if needed. Add more chocolate chips or chunks on if desired.
Cool completely, then cut into squares. Serve and ENJOY! Store at room temperature in an air tight container for up to 5 days, if they last that long!