*Note: the recipe notes have more cookie flavors. Mix & match any 4 flavors you want!
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, melted and slightly cooled, 1 1/4 cup brown sugar, packed (dark or light, I tested with both), 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, room temperature, 1 large egg yolk, room temperature, 2 Tbsp. whole milk, 1 1/2 Tbsp. vanilla extract
Add the dry ingredients.
3 cups all purpose flour, spooned and leveled, 1 1/2 tsp. kosher salt, 1 tsp. cornstarch, 1 tsp. baking soda, 1 tsp. baking powder
Fold until mostly combined. Do not over mix the cookie dough in this step.
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.
Sprinkle a little bit of the cinnamon/sugar mixture on the snickerdoodle section, and add additional sprinkles on the birthday cake section. (You'll add chocolate chunks or chips on AFTER the other flavor cookies bake).
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!