Into a big mixing bowl, add the room temperature butter, and cream it with an electric hand mixer for 2-3 minutes or until it’s creamy and light in color.
Add the brown sugar, and granulated sugar. Cream the butter and sugar with an electric hand mixer until the mixture is creamy and light.
Add the egg, kosher salt, vanilla extract, and mix until combined and creamy.
Add the flour, cake flour, cornstarch, baking soda, and mix with the hand mixer until almost combined.
Add the chocolate chunks and the m&m’s and fold with a spatula until the mixture is just combined.
Cover the bowl with plastic wrap and let the dough rest in the fridge for at least an hour. We let the dough rest in the fridge overnight.
Use a cookies scooper to scoop out the cookies or scoop the cookies with a regular tablespoon. For giant cookies like we made, weigh 6 oz each. For regular-sized cookies, scoop 1 ½ tablespoon of the cookie dough.
Place the cookie dough balls onto a parchment paper-lined baking sheet, and place extra m&m’s on each cookie. Bake for 11-12 minutes for the regular-sized cookies, and 14 minutes for the giant ones.
Let the cookies cool for 10 minutes, and then transfer them to a wire rack to cool completely.