Preheat a convection oven to 350°F (175°C) and line two heavy-gauge baking sheets with silicone mats or parchment paper.

In a food processor, pulse the Lucky Charms cereal until it reaches a fine, powdery consistency.

In a stand mixer fitted with the paddle attachment, cream the softened butter and granulated sugar on medium-high speed until light and aerated, approximately 3 minutes.

Add the egg and vanilla extract, beating until fully emulsified.

Sift together the all-purpose flour, cereal flour, baking soda, and salt in a separate vessel.

Reduce mixer speed to low and gradually incorporate the dry ingredients into the wet mixture until no streaks of flour remain.

Manually fold in 3/4 cup of the dehydrated marshmallows using a spatula to prevent breakage.

Portion the dough into 25g spheres and place them 2 inches apart on the prepared baking sheets.

Press the remaining 1/4 cup of marshmallows into the surface of each dough ball.

Bake for 9-10 minutes until the edges are set but the centers remain pale and soft.

Allow the cookies to temper on the baking sheet for 5 minutes before transferring to a wire cooling rack.