In a large bowl, beat softened butter, brown sugar, and granulated sugar together until light and fluffy, 1–2 minutes with a hand mixer (or a stand mixer with the paddle attachment). Add egg and vanilla extract, mixing until smooth.
In a separate bowl, whisk together flour, crushed cereal, baking soda, and salt.
Slowly add the dry ingredients to the wet ingredients and mix until just combined. Stir in ½ cup of the marshmallows with a silicone spatula.
Scoop dough into 1.5 to 2 tablespoon-sized balls. Place on a parchment-lined baking sheet and chill for 30 minutes or overnight.
If chilled overnight, gently press each dough ball to about ¾-inch thick before baking to encourage even spreading.
Preheat oven to 350°F. Bake for 8–10 minutes, or until edges are lightly golden and centers look slightly underbaked.
Immediately press the remaining marshmallows into the tops of warm cookies.
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack.