Instructions

1. Heat a large nonstick skillet over medium heat. Add 6 cups rice crisp cereal and cook, stirring occasionally, until lightly toasted and fragrant, 5 to 7 minutes. Remove from the heat and set aside.
2. Melt 6 tablespoons unsalted butter in a large saucepan or pot over low heat, gently stirring with a rubber spatula so all the butter melts evenly. Once the butter begins to foam, watch carefully, as it will quickly begin to brown and emit a nutty aroma. When browned, turn off the heat. Stir in 1/4 teaspoon kosher salt and 1/2 teaspoon vanilla extract if using.
3. Working quickly, add half the marshmallows (about 2 cups) and stir until thoroughly melted. Add the remaining marshmallows (about 2 cups) and stir until melted. The residual heat from the butter should be enough to melt them off the heat, but you can return the pot to low heat for a few seconds if the marshmallows aren't melting easily. Keep a close eye on the butter, however — you don't want it to continue to brown to the point of burning.
4. Add the toasted cereal and gently fold it in until the cereal is completely coated with the marshmallow mixture.
5. Transfer the mixture to an 8x8-inch baking pan. Using the rubber spatula, lightly and gently press into an even layer.
6. Let the treats stand at room temperature until set, about 1 hour. Cut into 2-inch squares and serve.