Dough. In a medium bowl use a whisk to combine together the flour and the baking soda. In a large mixing bowl, beat together the butter and sugars until creamy. Then add the egg and vanilla extract and mix to combine. On low speed, gradually add the dry ingredients from the first bowl and mix until just combined. Lastly, use a rubber spatula or similar item to fold in the chocolate chips.
Chill. Cover the dough and refrigerate it for at least an hour.
Prepare. When you are ready to bake your cookies, prepare your baking sheet for nonstick and preheat your oven to 350° F. Parchment paper or a baking mat should work in this recipe.
Form. Use a scoop or similar item to scoop out a quarter cup of the cookie dough and roll the dough between your hands to shape it into a ball. Place the ball on to your prepared baking sheet and repeat this step with the remaining cookie dough, keeping the dough balls about three inches apart from one another.
Stuff. Use the palm of your hand or the bottom of a cup to press each of the cookie dough balls into circles about a half inch tall. Then place one or two small Hershey bar squares (depending on how much chocolate you want inside) in the center of each cookie. On top of the chocolate place a marshmallow. Lastly, press the sides of the cookies up around the marshmallows, leaving the marshmallow tops exposed.
Bake. Bake for eight to 10 minutes, or until the bottoms of the cookies begin to turn golden brown and the marshmallows flatten out and become gooey.
Set. After taking the cookies out of the oven, allow them to sit on the baking sheet for 10 minutes to set. Then transfer the cookies to a cooling rack to fully cool. While the cookies are still warm you can add sprinkles and candy eyeballs on top of the melted marshmallows.