Lightly grease a 9×13-inch baking dish with butter or nonstick spray, or line it with parchment paper, leaving some overhang for easy lifting.

Pour 6 cups of crisp rice cereal into a large mixing bowl and set aside. Measure 10 cups of mini marshmallows, reserving 2 cups to stir in later and 8 cups for melting.

In a large, heavy-bottomed pot or Dutch oven, melt 6 tablespoons of unsalted butter over low heat. Keep the heat low to avoid scorching and tough treats.

Add 8 cups of mini marshmallows to the melted butter. Stir constantly over low heat for about 3–5 minutes, until almost fully melted and smooth with just a few tiny lumps remaining. If the mixture starts bubbling aggressively, reduce the heat.

Turn off the heat. Stir in the vanilla extract, fine sea salt, and the optional 2 tablespoons of heavy cream or half-and-half until fully combined.

Pour the warm marshmallow mixture over the cereal in the large mixing bowl. Add the remaining 2 cups of mini marshmallows. Using a buttered spatula or sturdy wooden spoon, stir until all of the cereal is evenly coated.

Transfer the mixture to the prepared baking dish. Lightly butter your hands or use a piece of parchment paper to gently press the mixture into an even layer. Do not press too firmly or the bars will become dense and hard.

While the top is still slightly warm, add sprinkles, a light pinch of flaky sea salt, or drizzle with melted chocolate or colored candy melts if desired. Let the pan cool at room temperature for 30–45 minutes, or until set.

Lift the slab out of the pan using the parchment overhang, if using. Cut into 18–24 bars with a sharp knife, wiping the knife between cuts for clean edges. Serve and enjoy.