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

In a medium saucepan, melt ½ cup salted butter over medium heat until golden brown, about 5-7 minutes.

Combine the cooled brown butter with ½ cup brown sugar and ¼ cup granulated sugar in a large mixing bowl.

Add the egg and 1 teaspoon vanilla extract; blend until smooth and creamy.

In a separate bowl, whisk together 1 teaspoon baking soda, ½ teaspoon salt, and 1 ½ cups flour.

Gradually add the dry mixture to the wet ingredients, mixing gently with a spatula.

Fold in 2 cups Rice Krispie cereal and 1 cup mini marshmallows until evenly distributed.

Scoop 3-tablespoon portions of dough onto the baking sheet, spacing them about 2 inches apart.

Bake for 10-13 minutes until edges are golden and centers are slightly soft.

Remove from oven, sprinkle with flaked salt, and cool on the sheet for 5 minutes before transferring to a wire rack.