Preheat oven: Set your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Cream butter and sugars: In a large mixing bowl, beat together butter, brown sugar, and granulated sugar until light and fluffy.
Add egg and vanilla: Mix in the egg and vanilla extract until smooth.
Combine dry ingredients: In another bowl, whisk flour, baking soda, and salt. Slowly add this dry mixture to the wet mixture until just combined.
Fold in mix-ins: Gently stir in rice krispie cereal and mini marshmallows. The marshmallows will melt slightly while baking, giving you that gooey center.
Shape and bake: Scoop tablespoon-sized balls of dough onto your prepared baking sheets, spacing them 2 inches apart. Bake for 10–12 minutes until the edges are golden.
Cool: Let the cookies cool on the pan for 5 minutes before transferring to a wire rack. This step helps the marshmallows set and keeps the cookies chewy.
