Preheat oven to 350 degrees. Line baking sheet with parchment paper.
In a large bowl, beat together softened butter and sugars until fluffy.
Then beat in egg and vanilla.
In a separate small bowl, whisk together flour and baking soda.
Beat dry ingredients into the wet mixture until well combined.
Fold in marshmallows.
Bake for 8-10 minutes.
Press additional marshmallows into the tops of the cookies right after removing from the oven.
