Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.
- In a large bowl, cream together the softened butter, peanut butter, brown sugar, and granulated sugar until light and fluffy (about 2-3 minutes) using a hand mixer or stand mixer. Add the egg and vanilla extract, and mix until fully combined.
- In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until combined.
- Gently fold in the white chocolate chips, mini marshmallows, and crispy rice cereal until evenly distributed throughout the dough.
- Scoop about 1-2 tablespoons of dough and roll it into a ball. Place the dough balls about 2 inches apart on the prepared baking sheets. Gently flatten each dough ball slightly with your fingers or the back of a spoon.
- Bake the cookies for 8-10 minutes, or until the edges are golden brown but the centers are still soft. Be careful not to overbake.
- Allow the cookies to cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.