Prep your pan. Line a 9x13-inch baking pan with parchment, leaving some overhang for easy lifting.
Lightly grease the parchment and a rubber spatula.
Brown the butter. In a large pot over medium heat, melt 8 tablespoons (1 stick) unsalted butter. Keep cooking, stirring, until the milk solids turn golden and smell nutty, about 4–6 minutes. Don’t walk away; it turns quickly.
Make a quick caramel. Reduce the heat to low.
Add 1/2 cup packed light brown sugar and 1/4 cup heavy cream to the browned butter. Stir constantly until smooth and gently bubbling, 2–3 minutes.
Fold in marshmallows. Add 10 ounces of mini marshmallows. Stir over low heat until fully melted and glossy.
Remove from heat and stir in 2 teaspoons vanilla and 1/2 teaspoon kosher salt.
Mix with cereal. Add 6 cups crispy rice cereal. Stir gently with the greased spatula until every bit is coated. If using chocolate chips, let the mixture cool 1–2 minutes first so they don’t fully melt.
Press into the pan. Transfer to the prepared pan.
Lightly press into an even layer with the greased spatula or a sheet of parchment. Don’t compact too hard or the bars will be dense.
Finish with flaky salt. While still warm, sprinkle 1–2 teaspoons flaky sea salt evenly over the top. Add a drizzle of melted chocolate if you like.
Set, then slice. Let cool at room temperature for 45–60 minutes, or chill for 20–30 minutes for cleaner cuts.
Lift out by the parchment and slice into squares or bars.