Preheat the oven to 325°F. Spray a standard half sheet pan with nonstick spray (13x18) and set aside.
Add the melted butter and sugars together in a large mixing bowl and whisk together until smooth. Be sure to break up any clumps of brown sugar.
1 cup unsalted butter, melted and slightly cooled, 1 1/2 cups dark brown sugar, packed, 1 cup white sugar
Add the eggs, milk and vanilla.
2 large eggs, room temperature, 2 large egg yolks, room temperature, 2 Tbsp. whole milk, 1 1/2 Tbsp. pure vanilla extract
Whisk until smooth.
Add the dry ingredients.
3 1/2 cups all purpose flour, spooned and leveled, 2 tsp. cornstarch, 1 tsp. baking soda, 1 tsp. baking powder, 1 1/2 tsp. kosher salt
Fold until mostly combined.
Add the chocolate chips in and fold just until combined.
2 cups chocolate chips, plus more to sprinkle on top
Turn the batter out onto the prepared baking sheet.
Spread evenly until smooth. Trust the process, it will all spread out and fit on the pan 🤗 Top with more chocolate chips if desired, or you can sprinkle a few on top AFTER it bakes so they get melty on the tops post baking.
Bake for ~22 minutes. 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.
Cool completely. Cut into portions and enjoy!