To start, you want to combine the butter and sugars. Mix until fluffy, about 5 minutes.
Add in one egg and mix and then the yolk and mix well. Then the vanilla and mix well.
If you're using a stand mixer, dump in the dry ingredients with the marshmallows and chocolate chips. Pulse until just combined. Then finish mixing with a rubber spatula. If you're using a hand mixer, dump the dry ingredients in and mix slightly until crumbly. Add in the marshmallows and chocolate and mix until just combined. Finish mixing with a rubber spatula. Don't overmix.
Cover and chill for at least 15 minutes, but up to overnight.
Use an ice cream scoop to scoop out the dough on to a greased baking sheet and round them. Press down slightly and place about 2" apart.
Bake at 350°F for about 10-11 minutes or until the outside edges are lightly browned. Take them out of the oven and leave the cookies on the sheet for another five minutes before moving to a wire rack.