Preheat your oven to 325°F. Line a half-sheet pan (around 13x18 inches) with parchment paper for easy removal and set aside.
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
1 cups unsalted butter, 1 & ½ cups light brown sugar, ½ cup granulated sugar
Beat in the eggs, corn syrup, and vanilla extract until combined.
3 large eggs, 1 tablespoon cornsyrup, 2 tablespoons pure vanilla extract
Mix in the flour, baking soda, baking powder, cornstarch, and salt, until just combined. Don't over-mix!
3 ¼ cups all-purpose flour, 1 tablespoon cornstarch, ¾ teaspoon baking soda, ¼ teaspoon baking powder, 1 teaspoon salt
Gently fold in the chocolate chips and M&Ms using a rubber spatula.
1 cups semi-sweet chocolate chips, 1 cups red white and blue M&Ms
Spread the dough evenly into the prepared pan. I like to use an offset spatula spatula to spread it evenly.
Bake for 20-23 minutes, or until the edges are golden brown and the center is just set (it may look a little soft, but it will firm up as it cools).
Let the bars cool completely in the pan on a wire rack before cutting into squares. Enjoy with a tall glass of milk!