Heat the Oven. Preheat oven to 350 degrees F. Line a 9x13-inch metal baking pan with foil. Grease the foil lightly with nonstick cooking spray.
Make the Base. (If using store-bought dough, skip to next step.) Whisk together flour, baking soda, and salt. Set aside. Combine butter, brown sugar, granulated sugar, and vanilla extract in a large bowl. Cream until smooth. Add eggs, one at a time, until well blended. Stop mixer. Add whisked dry ingredients. Mix until a dough forms. Stop the mixer again.
Add Chocolate Chips and Marshmallows. Stir the chocolate chips and 2 cups marshmallows into the dough. (Or just marshmallows if you're using premade dough.)
Bake. Press dough into prepared pan. Bake until edges are golden brown, about 20 minutes. Remove pan from the oven. Allow to cool. As the bars cool, the marshmallows will melt, leaving some holes in the bars. This is normal.
Toast Marshmallows. Place pan in a large rimmed baking sheet. Top with remaining marshmallows. Broil until marshmallows are toasted, about two minutes. This happens very quickly. Keep your eye on the pan to prevent burning. Allow bars to cool. Cut into squares. Serve. Store bars wrapped on the counter for up to three days.