In a small bowl, melt the butter and cool in the fridge for about 15 minutes. Separate 2 cups of Lucky Charms marshmallows from the cereal into a bowl.
1 cup (226 g) unsalted butter, melted and cooled
Set aside for now. In another bowl whisk together the flour, cornstarch, baking powder, baking soda and salt until well combined.
2½ cups (300 g) all purpose flour, 1 teaspoon cornstarch, 1 teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large mixing bowl, whisk the cooled melted butter and granulated sugar until well combined. Whisk in the egg, egg yolk and vanilla extract until smooth and well combined.
1½ cups (300 g) granulated sugar, 1 large egg, room temperature, 1 large egg yolk, room temperature, 1 teaspoon vanilla extract
Stir the dry ingredients into the wet, followed by 1 cup of Lucky Charms marshmallows until just combined. Reserve the remaining cup of marshmallows for topping the cookies right after baking. If you would like to add sprinkles or white chocolate chips, do so now.
2 cups (68 g) Lucky Charms marshmallows, divided
Using a medium cookie scoop (2 tablespoons, 30 ml), scoop the cookie dough into balls. Roll the cookie dough into sugar and place onto a lined large plate or small baking sheet. Cover the cookie dough balls and transfer to chill in the fridge for at least 30 - 60 minutes.
¼ cup (50 g) sugar for rolling, optional
Preheat the oven to 375 F (190 C) for about 20-30 minutes before baking the cookies. Line a large baking sheet with parchment paper. Remove about 8 cookie dough balls from the fridge onto a lined baking sheet about 3 inches ( 8 cm) apart.
Bake at 375 F (190 C) for 10 -12 minutes or until the edges of the cookies are set and are a light golden brown around the edges.As soon as the cookies come out of the oven, top with more marshmallows from the reserved cup of marshmallows. Nudge any misshapen cookies back into a round shape using a cup, bowl or biscuit cutter that is slightly larger than the cookies. Cool the chewy Lucky Charms marshmallow cookies on the baking sheet for 3 - 4 minutes. Then transfer to a cooling rack to cool completely to room temperature and enjoy!