• Lightly butter an 8-inch / 20cm square cake pan.
• In a large heavy pot over medium-low heat, bring the butter and salt to a simmer. Turn off the heat and add the mini-marshmallows.
• Stir until the mixture is smooth. Try not warm it up any more, or your bars will end up hard. If the marsh-mallows are not fully melted, cover the pot and let them sit a minute. Stir in the vanilla, add the rice cereal, and mix until thoroughly incorporated.
• Pour the mixture into the prepared pan. Allow to set atroom temperature for about 30 minutes before cutting.

Brown Butter

• In a small saucepan over medium-low heat, melt the butter. Turn the heat to medium-high and boil until the butter is frothy, about 3 minutes (longer if butter is chilled or frozen). Turn the heat to low and simmer, without stirring, until the milk solids have settled to the bottom of the pan, are dark golden brown, and smell slightly toasted, another 15 to 18 minutes, depending on how dark you like your toast (I like mine on the darker side). Once the milk solids start to caramelize, it goes quickly, so don’t walk away. Remove from the heat once the butter is a deep amber color. Strain through a fine-mesh sieve, if desired.
• Allow the butter to cool completely, then cover and refrigerate for up to 2 weeks or freeze for up to 2 months.