Preheat oven to 350 degrees.
In a medium bowl, combine flour, baking soda and salt and set aside.
Using a stand mixer (or handheld mixer - your choice!), beat the softened butter and the granulated and brown sugars together. You want to mix the butter and sugars for about 2 minutes until the mixture is a light, consistent color.
Add the eggs and vanilla and beat for another 1-2 minutes while scraping the sides of the bowl.
Reduce your mixing speed to low and slowly add the flour mixture and mix until just combined and smooth. No need to over beat it. Stir in 1/2 cup mini chocolate chips.
Press dough into an 8" cast iron skillet. NOTE: If you use larger or smaller, it will affect your baking time.
Bake cookie for 8 minutes at 350 degrees. Carefully, remove skillet from oven and top with chopped Reese's Peanut Butter Cups (or Reese's Minis) and mini marshmallows.
Return skillet cookie to the oven and cook for an additional 18 minutes. NOTE: Use a knife to check to see that the center isn't too raw. You want the cookie to be slightly browned on the edges but not raw in the center. Depending on your size skillet, you may need to add additional cooking time.
Remove from oven and let sit for about 20 minutes and cool slightly before serving.