Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In a large mixing bowl, beat the butter and sugar for 2-3 minutes until creamy and light. Stop and scrape the bottom and sides of the bowl.
1 cup salted butter, 1 1/4 cups granulated sugar
Add the eggs and vanilla, mixing until combined.
2 large eggs, 2 teaspoons pure vanilla extract
Add the flour, cornstarch, baking soda, and salt. Stir on medium-low speed until combined and only a few streaks of flour remain.
2 3/4 cups (388g) all-purpose flour, 1 teaspoon cornstarch, 3/4 teaspoon baking soda, 1/2 teaspoon salt
Stir in the white chocolate chips and Lucky Charms marshmallows and cereal pieces on low speed or by hand until evenly dispersed.
1 1/2 cups lucky charms cereal, 1 cup white chocolate chips
Use a medium cookie scoop to place balls of cookie dough on the prepared baking sheets, spacing them a few inches apart so they have room to spread while they bake. Bake for 10-12 minutes until set around the edges. Immediately press a few extra marshmallow pieces into each of the warm cookies right after removing them from the oven. Let set up for 5 to 10 minutes on the baking sheet before transferring to a cooling rack.