1 ounce (2 tablespoons) unsalted butter
3.5 ounces (about 2 cups) miniature marshmallows
1-1/2 cups crisped rice cereal (such as Rice Krispies)
1/8 teaspoon vanilla extract
Small pinch kosher salt
Browned butter-marshmallow rice crisp bars
Coat a 4-inch square pan with cooking spray or a light coating of butter. (You can also use an individual spring form pan; if it’s non-stick, you don’t need to coat it.)
Place the butter in a saucepan large enough to hold the marshmallows and cereal and melt it over medium heat. Continue to cook as it foams up and then clears. As the milk solids start to brown, stir the butter, scraping the bottom (they may start to brown while the butter is still foaming, so stir and check often). Cook just until the solids are all deep golden brown but not burned; this should happen quickly once the butter begins to brown.
Take the pan off the heat and stir in the marshmallows, the vanilla extract and the salt. Stir until the marshmallows have melted and the mixture foams up a bit. Add the cereal to the browned butter-marshmallow mixture and stir until the cereal is completely coated.
Pour out into the prepared pan. Spread the mixture out into the corners of the pan and pack down firmly. (If you don’t have a pan small enough, just spoon the mixture onto a small buttered or oiled plate or sheet pan and form into a square about 4 by 4 inches.)
Optional flavor step:
Before you start browning the butter, measure out about half a cup of the marshmallows. Line a small cookie sheet or sheet pan with parchment paper and arrange the marshmallows in a single layer but very close together. Turn the broiler on.
Just before mixing the cereal into the marshmallow-butter mixture, place the sheet pan in the oven and broil just until the marshmallows are nicely browned but not burned. Depending on the strength of your broiler and the placement of the rack, this can take anywhere from 30 to 90 seconds. While the marshmallows are toasting, mix the cereal into the butter-marshmallow goo. As soon as the marshmallows are toasted, remove the pan from the oven and scrape the marshmallows into the cereal mixture. Don’t worry if you leave behind the bottoms of the toasted marshmallows; you’re more interested in the toasty tops. Stir them into the cereal mixture and form into bars as described above.