Spray an 8×8-inch pan (or 9x9 for slightly thinner bars) with non-stick spray and line it with foil or parchment, with a bit of overhang. Spray generously again. Line a baking tray with foil and spray generously with non-stick spray.

Turn the oven on to broil and make sure a rack is in the center (not too close to the heat). Separate 2 cups of the marshmallows and set them aside. Add the rest (10 cups) to the sprayed foil lined sheet. Place the marshmallows in the oven until they turn golden brown and toasted on top (30 seconds to about 90 seconds). Watch them closely because they go from nicely toasted to charred very quickly! Remove marshmallows from the oven and let cool while you prepare the brown butter.

Melt the butter in a large, heavy pot set over medium-low heat. Once it starts to foam, use a silicone spatula to stir and scrape the bottom of the pot as the butter turns golden and dark flecks begin to form. The butter should turn golden brown and smell nutty (be careful to not let it burn. Once it starts to brown, it will darken very quickly).

As soon as the butter is brown, turn the heat to low and add the toasted marshmallows. Stir until the marshmallows are almost completely melted (a few lumps are fine). Then turn off the heat and stir in the salt, vanilla and rum (if using).

Stir in the cereal, pretzels and reserved marshmallows. When mostly combined, add in the white chocolate chips and stir until completely combined.

Working quickly, transfer to the prepared pan and very gently press until the top is flat and even (*use the reserved butter wrapper to keep your fingers from sticking, or spray a spatula). Don’t press too firmly or treats will get hard. Allow to cool to room temperature before cutting them into squares.