Place the sliced butter in a light-colored skillet or saucepan over medium heat. (Light-colored helps you determine when the butter begins browning.) Stir or whisk the butter constantly as it melts. Once melted, the butter will begin to foam. Keep stirring. After 5–8 minutes, the butter will begin browning—you’ll notice lightly browned specks begin to form at the bottom of the pan and it will have a nutty aroma. As soon as the butter has browned, immediately remove from heat and pour it into a medium heat-safe bowl. Scrape up the browned solids at the bottom of the skillet and add them as well. Set aside to slightly cool. After browning, you should have around 185-190g butter.

In a large bowl, whisk together the flour, cornstarch, baking soda, and salt. Fold in the rice cereal and mini marshmallows.

Whisk the brown butter, brown sugar, granulated sugar, egg, egg yolk, and vanilla together until combined. Pour into the flour mixture and stir with a silicone spatula until fully incorporated. The dough will be greasy and a bit crumbly; that’s ok.

Scoop the dough, about 2 Tablespoons (40g) per cookie, and use your hands to form it into a ball. Squeeze firmly to help it hold its shape. Place the dough balls on a parchment-lined baking sheet and freeze, uncovered, for 1 hour. If freezing for longer (up to 1 day), transfer the firm dough balls to an airtight container or zip-top bag.

Preheat the oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Arrange the frozen dough balls 3 inches apart on the prepared baking sheets. Bake for 14-15 minutes, or until the cookies are lightly browned on top, the edges look crisp, and the centers still look soft. (If the cookies haven’t begun to spread by the 10-minute mark, remove the baking sheet from the oven, give it a few firm taps on the counter, then return it to the oven.) Some edges may over-spread if a marshmallow is on the outside. This is normal and expected.

Sprinkle sea salt, if using, on top of the warm cookies right after you remove them from the oven. Let the cookies cool on the baking sheet for 10 minutes before transferring to a cooling rack to cool completely.

Cover cooled cookies tightly and store at room temperature for up to 1 week.