Preheat your oven to 350°F (175°C).
Line your 9x13 inch baking pan with parchment paper, leaving some overhang on the sides.
In a large mixing bowl, combine the melted butter and light brown sugar. Whisk until smooth, then add the eggs one at a time, followed by the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, cornstarch (if using), and salt.
Gradually add the dry mixture to the wet ingredients, mixing until just combined.
Gently fold in the mini marshmallows, semi-sweet chocolate chips, and caramel chips using a spatula.
Spread the cookie dough evenly into the prepared baking pan. Bake for 25-30 minutes, or until the edges are golden brown.
Cool the bars in the pan for about 15 minutes, then lift them out using the parchment paper and allow to cool completely before cutting into squares.
