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

In a medium saucepan over medium heat, melt ½ cup of salted butter, stirring continuously until it turns golden brown.

In a large mixing bowl, combine the cooled browned butter, brown sugar, and granulated sugar, mixing until smooth. Add the egg and vanilla extract, mixing until blended.

Gradually sift in the all-purpose flour, baking soda, and salt into the wet mixture, stirring gently until combined.

Fold in the Rice Krispie cereal and mini marshmallows until evenly distributed.

Using a 3-tablespoon cookie scoop, form dough balls and place them on the prepared baking sheet, leaving space for spreading.

Bake the cookies for 10-13 minutes until the edges are light golden brown.

Sprinkle flaked salt on top of each cookie and allow to cool on the baking sheet for about 5 minutes before transferring to a wire rack.