Lightly grease a 9x13-inch baking pan with butter or nonstick spray. For easier removal and clean cuts, line the pan with parchment paper, leaving some overhang as handles, then grease the parchment as well.

Add 6 cups of crisped rice cereal to a large mixing bowl and set aside. Measure 3 cups of mini marshmallows into a separate small bowl. Having everything ready before melting the butter prevents the mixture from firming up while you work.

In a large pot or Dutch oven, melt 6 tablespoons of butter over low heat. For a deeper, nutty flavor, you can cook the butter a few minutes longer until golden brown and fragrant, keeping the heat low and watching carefully.

Stir 7 cups of the mini marshmallows into the melted butter. Keep the heat on low and stir often with a heatproof spatula until the marshmallows are almost fully melted and smooth. Remove from heat just before the last small lumps disappear to avoid overheating, which can make hard bars.

Off the heat, stir in 1 teaspoon vanilla extract and 1/4 teaspoon fine sea salt. Taste a tiny bit carefully and adjust the salt if needed so the mixture tastes pleasantly sweet but balanced.

Working fairly quickly, pour the warm melted marshmallow mixture over the cereal in the large bowl. Fold gently with a spatula, scooping from the bottom, until all the cereal is coated without crushing it.

When the cereal is mostly coated and still warm and sticky, gently fold in the reserved 3 cups of mini marshmallows. They should stay mostly intact, creating gooey, stretchy pockets in the finished bars.

Transfer the mixture to the prepared pan. Lightly grease your hands or a spatula and gently press the mixture into an even layer. Do not pack it down firmly—use light, even pressure so the bars stay soft and chewy.

Let the treats cool at room temperature for 30–45 minutes, or until set enough to cut cleanly. If needed, you can chill them in the fridge for about 20 minutes to speed up setting, but avoid chilling for hours so they don’t become too firm.

Use the parchment handles to lift the slab from the pan. With a sharp knife, cut into 16–24 squares, wiping the blade between cuts for neat edges. Serve and enjoy, ideally while they are still slightly warm.