Preheat your oven to 350°F (175°C). This ensures the cookies bake evenly.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar using a whisk or electric mixer until light and fluffy, about 3-5 minutes. This process incorporates air into the mixture, helping the cookies rise.

Add the eggs one at a time, mixing well after each addition. Then stir in the vanilla extract. This adds moisture and flavor to the dough.

In a separate bowl, whisk together the sifted flour, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined. Be careful not to overmix, as this can lead to tough cookies.

Gently fold in the crispy rice cereal, chocolate chips, and marshmallows into the cookie dough using a rubber spatula. Ensure everything is evenly distributed.

Line a baking sheet with parchment paper. Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers may look slightly underbaked, but they will continue to cook on the baking sheet after being removed from the oven.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. This helps them set and maintain their chewy texture.