Preheat your oven to 350°F (175°C) and line a large baking sheet with parchment paper or a silicone baking mat.

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.

Add the egg and vanilla extract, beating until fully incorporated. Scrape down the sides of the bowl to ensure even mixing.

In a separate bowl, whisk together the flour, baking soda, baking powder, and salt.

Slowly add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Gently fold in 1 cup of the Lucky Charms marshmallows by hand.

Use a size 40 cookie scoop to form 2-tablespoon sized balls. Place them 2 inches apart on the baking sheet.

Bake for 9-11 minutes, or until the edges are barely golden and the tops look set.

Remove from the oven and immediately press the remaining 1 cup of marshmallows into the tops of the warm cookies.

Cool on the pan for 5 minutes before transferring to a wire rack to cool completely.