Start by preheating the oven to 350F, then line a large baking sheet with parchment paper and set aside.
In a stand mixer cream the butter, granulated sugar, and brown sugar together using a paddle attachment until light in color and fluffy. Add in the egg and the vanilla and mix again for 30 seconds or until fully incorporated.
½ cup butter, ½ cup granulated sugar, ¼ cup brown sugar, 1 large egg, 1 teaspoon vanilla
Add in the flour, cornstarch, baking soda, baking powder, and salt and mix just until the flour disappears, then fold in the lucky charms marshmallows. Using a small cookie scooper, scoop out equally sized cookie dough balls, roll them in the marshmallows, and place them on the parchment-lined baking sheet. Make sure to leave 2 inches between each cookie because they will spread.
1 &⅓ cup + 1 tablespoon all purpose flour, 2 teaspoon cornstarch, ½ teaspoon baking soda, ¼ teaspoon baking powder, ¼ teaspoon salt, 1 cup lucky charms marshmallows
Bake the cookies for 8 minutes, pull them out and let them rest on the cookie sheet for 5-10 minutes then transfer to a cooling wrack. Enjoy immediately or store in an airtight container.