Step 1: Preheat your oven to 350°F (175°C) and line a large baking sheet with parchment paper or a silicone baking mat.
Step 2: In a stand mixer with the paddle attachment, cream the softened butter and granulated sugar on medium-high speed for 2-3 minutes until the mixture is pale and fluffy.
Step 3: Add the egg and vanilla extract, beating until fully incorporated. Scrape down the sides of the bowl to ensure even mixing.
Step 4: In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.
Step 5: Slowly add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Gently fold in 1 cup of the Lucky Charms marshmallows by hand.
Step 7: Use a size 40 cookie scoop to form 2-tablespoon sized balls. Place them 2 inches apart on the baking sheet.
Step 8: Bake for 9-11 minutes, or until the edges are barely golden and the tops look set.
Step 9: Remove from the oven and immediately press the remaining 1 cup of marshmallows into the tops of the warm cookies.
Step 10: Cool on the pan for 5 minutes before transferring to a wire rack to cool completely.