Preheat the oven to 350 degrees Fahrenheit.
In a large mixing bowl, beat together the butter and brown sugar with a stand up mixer or hand held until the mixture is light and fluffy.
Add in the egg and vanilla extract and mix until well combined.
Reduce the speed on the mixer to low and slowly mix in the flour, salt and baking soda until just combined.
Make sure to regularly scrape down the sides of the bowl to ensure that any excess flour is mixed into the dough.
Set aside some of the marshmallows for topping and stir the remaining marshmallows into the batter.
Use a large cookie scoop (approximately 2 tablespoons) to scoop out the cookies and place on cookie sheets leaving about 2 inches between each cookie.
Press the remaining marshmallows into the top of each cookie. I like to top each cookie with 2-3 marshmallows.
Bake for 12-14 minutes on the top rack in the oven so that the marshmallows on top of the cookies are toasted slightly as they bake in the oven. Bake until the edges of the cookies are slightly golden brown.
Allow the cookies to set on the baking sheets for 5-10 minutes and then carefully move them to a cooling rack to cool completely.
Then they are ready to serve and enjoy!