Preheat the oven to 350°F and line a large baking sheet with parchment paper and set it aside.
In a bowl, combine all purpose flour, corn starch, kosher salt, and baking soda. Set aside.
2 cups + 2 tablespoons all purpose flour, 1 tablespoon corn starch , ¾ teaspoon baking soda , 1 teaspoon kosher salt
In a separate bowl, cream together unsalted butter, brown sugar, and granulated sugar, until smooth and creamy, about 3-4 minutes.
12 tablespoons softened butter , ¾ cup brown sugar , ½ cup white sugar
Mix in vanilla extract and optional cake batter extract until totally combined.
1 tablespoon vanilla extract , 1 teaspoon cake batter extract
Mix in the egg and the egg yolk until totally combined and smooth. Make sure to scrape the sides of the bowl to ensure everything is incorporated.
1 large egg, 1 large egg yolk
Sift in the dry ingredients. Mix gently to incorporate using a rubber spatula. When the dough is almost all of the way mixed (there may still be streaks of flour), fold in the Lucky Charms marshmallows. Make sure to scrape the sides of the bowl to ensure everything is incorporated.
¾ - 1.5 cups Lucky charms marshmallows
Use a large cookie scoop (#20) to get nice big balls of cookie dough.
Place the cookies on a baking sheet a few inches apart. This will make 12-14 cookies. We'd recommend chilling the cookies for 30 minutes - 2 hours, or even overnight if you have the time, though it is not entirely necessary and the cookies can be baked right away.
When it's time to bake, place 5-6 cookies on a baking sheet with plenty of room in between them so they can spread. Bake at 350°F until the edges are set, about 13-15 minutes. Let the cookies cool on the baking sheet for about 15 minutes, then transfer them to a cooling rack to cool completely before frosting.