Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, cream together the softened butter, granulated sugar, brown sugar, and vanilla extract until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Fold in the chocolate chips and mini marshmallows until evenly distributed.
Spread the cookie dough evenly in the prepared baking pan.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean.
Allow the bars to cool in the pan for at least 10 minutes before cutting into squares.