In a medium bowl, whisk together the all-purpose flour, salt, baking soda, and baking powder.
In a large bowl, use an electric mixer to cream the unsalted butter, optional shortening, and granulated sugar until light and fluffy, about 3-4 minutes.
Beat in the eggs and extra yolk, one at a time, mixing well after each addition. Stir in the Marshmallow Fluff and vanilla extract until smooth and creamy.
Gradually add the dry ingredients to the wet mixture, mixing just until combined. Avoid overmixing.
Gently fold in the crushed cereal pieces and marshmallow pieces. Add a few drops of green food coloring if desired and gently swirl.
Using a cookie scoop or tablespoon, drop dough onto prepared baking sheets about 2 inches apart. Press extra marshmallow pieces on top of each cookie.
Bake at 350°F (175°C) for 10-12 minutes, until edges are golden and centers are set. Cool on baking sheet for 5 minutes, then transfer to cooling rack.