Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper to prepare for the cookies.
In a medium bowl, whisk together the all-purpose flour, salt, baking soda, and baking powder. Set this mixture aside.
In a large mixing bowl, use an electric mixer to cream together the unsalted butter and granulated sugar until the mixture is light and fluffy. This should take about 2-3 minutes.
Beat in the large eggs and extra yolk, one at a time, mixing well after each addition. Then, add the vanilla extract and mix until combined.
Gradually add the dry ingredients to the wet mixture. Mix until just combined; be careful not to overmix.
Fold in the Marshmallow Fluff (if using), crushed Lucky Charms cereal, and marshmallow pieces until evenly distributed. If desired, add a few drops of green food coloring for a fun touch!
Using a cookie scoop or spoon, drop rounded tablespoons of dough onto the prepared baking sheet, leaving enough space between each cookie to allow for spreading. Top each cookie with a few extra marshmallow pieces.
Bake in the preheated oven for 10-12 minutes, or until the edges are golden and the centers are just set. The cookies will continue to cook slightly as they cool.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Enjoy your Lucky Charms Cookies with a glass of milk or your favorite beverage!