Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Melt ½ cup of salted butter in a medium saucepan over medium heat until golden brown, stirring continuously for 5-7 minutes. Let cool for about 10 minutes.

In a large bowl, combine the cooled brown butter, brown sugar, and granulated sugar, whisk until smooth. Mix in the egg and vanilla extract at low speed until combined and creamy.

Gradually sift in 1¾ cups of all-purpose flour, ½ teaspoon baking soda, and a pinch of salt. Mix gently until just incorporated.

Fold in 2 cups of Rice Krispie cereal and 1½ cups mini marshmallows until evenly distributed.

Using a 3-tablespoon cookie scoop, place 4-5 mounds of dough on the prepared baking sheet, leaving space for spreading.

Bake for 10-13 minutes, or until the edges are lightly golden.

Sprinkle flaked salt on top immediately after removing from the oven while still warm.