Preheat oven to 350 degrees F (175 degrees C). Line a 9x13-inch baking dish with foil and lightly spray with baking spray, set aside.
In a large bowl, beat Butter (12 tablespoon), Granulated Sugar (1 cup), Brown Sugar (2/3 cup) and Vanilla Extract (2 teaspoon) until light and fluffy, about 2 minutes.
Add Egg (2), one at a time, mixing until incorporated.
In another bowl whisk together All-Purpose Flour (1 1/4 cup), Unsweetened Cocoa Powder (3/4 cup), Baking Powder (1/2 teaspoon), Corn Starch (1 tablespoon) and Salt (1/4 teaspoon). Mix well so theres no lumps.
Add half of dry ingredients and half of Milk (3 tablespoon) to the batter and mix until combined then add the rest of dry ingredients and milk. Mix just until combined.
Stir everything gently a few times with a spatula to make sure it is all mixed completely.
With the spatula, Spread the batter evenly into prepared pan. Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean. Remove from the oven and allow to cool in the pan.
In the meantime, make the chocolate ganache by combining Semi-Sweet Chocolate Chips (1 cup), Milk Chocolate Chips (1/2 cup) and Heavy Cream (1/2 cup) in a bowl. Microwave for a minute or so. Whisk until chocolate is fully melted.
Spread on top of the brownies, sprinkle with desired topping such as mini M&M's, Rainbow Crunch Chips, etc.
Let the ganache set and then cut and enjoy!