Instructions

• Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside.
• In a medium bowl, whisk together 2 1/2 cups all-purpose flour, 1 1/2 teaspoons salt, 1 1/4 teaspoons baking soda, and 1/2 teaspoon baking powder. Set the dry mixture aside.
• In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl with a handheld mixer), beat 1 1/2 cups granulated sugar, 1 cup room-temperature unsalted butter, and 1/4 cup shortening (if using — or use the 1/4 cup additional butter if you chose that substitution) on medium-high speed for about 1 minute, until smooth and fluffy.
• Add 2 large eggs + 1 yolk, 1/3 cup Marshmallow Fluff (optional), and 1 teaspoon vanilla extract to the butter mixture. Beat on medium speed until fully incorporated and smooth.
• With the mixer on low, add the flour mixture and 2/3 cup crushed Lucky Charms cereal (cereal pieces only, no marshmallows). Mix until just combined; do not overmix.
• If making green marbled cookies: remove about 1/3 of the dough and place it in a separate bowl. Add a small amount of green food coloring to that portion and mix until evenly colored. If not making green marbled cookies, skip to step 7.
• To the large (plain/uncolored) portion of dough, fold in 1 1/2 cups marshmallow pieces from Lucky Charms cereal until evenly distributed. (For green marbled cookies, do not add marshmallows to the small colored portion; add marshmallows only to the plain portion.)
• Portion the dough: - Plain cookies: use a large 3-tablespoon scoop to portion 3-tablespoon balls of dough. Lightly flatten each ball into a roughly 2-inch disc. - Green marbled cookies: for each cookie, scoop 1 tablespoon of the green dough and 2 tablespoons of the plain dough. Place the two portions side-by-side and gently roll them together just until they stick but are not fully mixed. Slightly flatten to a roughly 2-inch disc.
• Place the dough discs on the prepared baking sheets about 3 inches apart to allow for spreading.
• Press several extra marshmallow pieces into the top of each disc for topping.
• Bake one sheet at a time on the center oven rack at 350°F for 18–20 minutes, rotating the sheet halfway through baking. Remove the cookies when the tops still look slightly glossy and set at the edges; avoid overbaking. (Optional: for flatter, chewier cookies, you can gently slam the baking sheet onto the oven rack a few times after the cookies have baked for about 10 minutes.)
• Let the cookies cool on the baking sheet for about 10 minutes, then transfer them to a wire rack to cool completely.
• Store cooled cookies in an airtight container at room temperature for up to 5 days or freeze for up to 4 months. Unbaked cookie dough balls can be frozen in an airtight container or zip-top bag for up to 4 months; when baking from frozen, add 1–2 minutes to the baking time.