Grease a loaf pan with butter or baking spray. Line the pan with parchment paper and set aside for later.

Then, melt the butter over medium-low heat. Let it gently simmer, swirling the pan often, until the butter smells nutty and there are small brown bits on the bottom of the pan.

Add the mini marshmallows to the brown butter. Stir the marshmallows constantly, just until they're melted and well-combined with the butter. Stir in the salt and vanilla extract. Remove the pan from the heat and add the puffed rice cereal. Stir until everything is well-combined.

Using a greased rubber spatula, gently press the mixture evenly into the prepared loaf pan. Then, let the bars set at room temperature or in the fridge for 30-60 minutes. Use the parchment paper to remove the treats from the pan. Slice into 6 bars and sprinkle with flaky salt if desired. Enjoy!