01 - Preheat oven to 350°F. Line baking sheets with parchment paper and set aside.
02 - In a large mixing bowl, cream together the softened butter, granulated sugar, and packed brown sugar using an electric mixer until the mixture is light and fluffy, approximately 2 to 3 minutes.
03 - Add the egg and vanilla extract to the creamed mixture, beating until fully combined and smooth.
04 - In a separate 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 just incorporated. Avoid overmixing to keep the cookies tender.
06 - Gently fold in the Rice Krispies cereal, mini marshmallows, and white chocolate chips using a spatula, taking care not to crush the cereal.
07 - Scoop rounded tablespoons of dough onto the prepared baking sheets, spacing each portion about 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 chewy texture.
09 - Allow the cookies to rest on the baking sheet for 5 minutes, then transfer to a wire cooling rack to cool completely before serving.