1. Preheat the oven to 350°F (175°C) and line two large baking sheets with parchment paper or silicone baking mats.

2. In a large mixing bowl, beat the softened unsalted butter, light brown sugar, and granulated sugar together until light and fluffy.

3. Add the eggs one at a time, mixing just until combined, then mix in the vanilla extract. Scrape down the sides of the bowl.

4. In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, fine sea salt, and cornstarch until well combined.

5. Add the dry ingredients to the wet ingredients in two additions, mixing on low speed just until no dry streaks of flour remain.

6. Fold in the semi-sweet chocolate chips, peanut butter chips, and mini marshmallows, reserving a small handful of each to press onto the tops of the cookies before baking.

7. Scoop the dough into balls using a medium cookie scoop (about 2 tablespoons of dough) and place them on a parchment-lined tray. Gently tuck any exposed marshmallows so they are mostly surrounded by dough.

8. Chill the dough balls in the refrigerator for 20–30 minutes to help the cookies bake up thicker and prevent excessive spreading.

9. Arrange the chilled dough balls on the prepared baking sheets, leaving several inches between each cookie. Press a few extra chocolate and peanut butter chips and marshmallows on top if desired.

10. Bake for 10–12 minutes, or until the edges are just turning golden and the centers still look slightly under-baked.

11. Allow the cookies to cool on the baking sheet for 5–10 minutes, then transfer them to a wire rack to cool completely.

12. Serve warm and gooey or at room temperature, and store any leftovers in an airtight container.