01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper and set aside.
02 - In a large mixing bowl, beat together the softened butter, granulated sugar, and packed brown sugar using an electric mixer on medium speed until the mixture is pale, light, and fluffy, approximately 2 to 3 minutes.
03 - Add the egg and vanilla extract to the creamed mixture. Continue mixing on medium speed until fully combined and smooth, scraping down the sides of the bowl as needed.
04 - In a separate medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt until evenly distributed.
05 - Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until the flour is just incorporated. Avoid overmixing to keep the cookies tender.
06 - Using a spatula, gently fold in the Rice Krispies cereal, mini marshmallows, and white chocolate chips until evenly distributed throughout the dough.
07 - Scoop rounded tablespoons of dough and place them onto the prepared baking sheets, spacing each portion approximately 2 inches apart to allow for spreading.
08 - Bake for 10 to 12 minutes, or until the edges are lightly golden and the centers appear just set. Do not overbake to maintain a soft, chewy texture.
09 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer them to a wire cooling rack to cool completely before serving or storing.