Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.

Brown the butter: melt the butter in a medium, light pan over medium heat. It will foam, then calm, then begin to turn golden and smell nutty. Stir constantly and pull it off the heat as soon as you see amber bits and smell toasty aromas. Pour the butter into a bowl and chill in the fridge or freezer for about 10 minutes so it cools but stays pourable. Watch closely — it can go from brown to burned in seconds.

In a mixing bowl whisk the cooled browned butter together with the brown and granulated sugars until smooth. Beat in the egg and vanilla on low speed until incorporated.

Add baking soda, salt, and the flour slowly, stirring until the dough just comes together. Don’t overwork it — you want a tender crumb.

Fold in the Rice Krispies gently, then fold in the mini marshmallows. The marshmallows will make the dough stickier — that’s normal.

Use a 3-tablespoon scoop (or two large spoons) to portion dough onto the prepared sheets. Arrange only 4–5 cookies per pan so they can spread without crowding.

Bake 10–13 minutes, until the edges turn light golden and the centers still look slightly soft. Avoid overbaking if you want a chewy interior. Immediately sprinkle with flaky sea salt.

Cool on a wire rack. These cookies crisp up a bit as they cool but keep a chewy marshmallow pocket.