Preheat oven to 350 F. Line a baking sheet with parchment paper and set aside.
In a large bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
In the bowl of stand mixer (or using a hand mixer), beat butter, sugar and brown sugar until light and fluffy, about 3 minutes. Add egg, egg yolk and vanilla, then mix until combined. Scrape down the sides and bottom of the bowl as necessary.
Add dry ingredients to the wet ingredients. Mix just until combined. Stir in 1 cup marshmallows by hand.
Use a medium cookie scoop to scoop dough into balls, about 1 ½ tablespoons. Roll into balls (avoid overworking the dough). Place on parchment-lined baking sheet spaced about 2 inches apart.
Bake at 350° F for 10 minutes or until the edges are set and starting to turn golden. As soon as the cookies come out of the oven, press a few more marshmallows on top. Cool on the pan for 5 minutes before transferring to a wire rack to cool completely.
Notes
Marshmallows: You will have to pick the marshmallows out of the cereal. I suggest buying two boxes to be safe. Alternatively, you can find just the marshmallows online and in select stores.Chilling the dough: For thicker cookies, chill the dough for 30 minutes to an hour. I do not recommend chilling the dough much longer because the marshmallows will break down and lose their texture.Store in an airtight container at room temperature for up to 5 days.Freeze the dough balls in a plastic bag for up to 3 months. Let them sit on a baking sheet at room temperature while the oven preheats. Bake for 12 minutes.