Separate 1.5 cups of the toasted oat cereal pieces from 1.5 cups of the marshmallows. Set the marshmallows aside.

Place the 1.5 cups of toasted oat pieces into a food processor or blender and pulse until ground into a fine powder (yields about 3/4 cup).

In a medium bowl, whisk together the all-purpose flour, the ground cereal flour, cornstarch, baking soda, and kosher salt. Set aside.

In a large mixing bowl or stand mixer, beat the softened butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy (about 2-3 minutes).

Add the whole egg, egg yolk, and vanilla extract. Beat until just combined.

Turn mixer to low and gradually add the dry ingredients, mixing only until just incorporated. Do not overmix.

Gently fold in 1 cup of the reserved Lucky Charms marshmallows using a spatula.

Cover the bowl tightly and chill the dough in the refrigerator for at least 1 hour (crucial for thick cookies).

Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

Scoop about 2-3 tablespoons of dough per cookie and roll into balls. Place 2 inches apart on the baking sheets. Press the remaining 1/2 cup of marshmallows onto the tops of the dough balls.

Bake for 10-12 minutes until edges are set but centers are still soft and slightly underbaked.

Remove from oven. If desired, use a large round cutter to swirl around the hot cookies to perfectly round them. Let cool on the pan for 10 minutes before moving to a wire rack.