Preheat the oven to 350 degrees F and line a 9-inch × 13-inch cake pan with aluminum foil or parchment paper and set aside.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.
In a large bowl with a hand mixer or (in a stand mixer fitted with a paddle attachment), cream together the butter, sugar, brown sugar, and vanilla until light and fluffy.
Add the eggs one at a time, mixing after each addition for about 10 seconds.
Add the dry ingredients to the butter mixture and mix just until combined. Fold in the chips, crushed pretzels, chocolate chips, M&M's, and most of the sprinkles.
Spread the cookie dough out evenly in the cake pan and top with extra M&M’s, chocolate chips, chips, pretzels, and the remaining sprinkles.
Bake the cookie bars for 22 to 25 minutes until the edges are golden brown. Remove from the oven and place on a wire rack.
Allow the bars to cool for at least 1 hour in the pan before removing and slicing them into squares.