Step 1

Toast the cereal. Heat a large nonstick skillet over medium heat. Add the cereal and cook, stirring occasionally, until lightly toasted and fragrant, 5 to 7 minutes. Remove from the heat and set aside.

Step 2

Brown the butter. Melt the 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 the salt and vanilla if using.

Step 3

Melt the marshmallows. Working quickly, add half the marshmallows and stir until thoroughly melted. Add the remaining marshmallows 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.

Step 4

Add the cereal. Add the toasted cereal and gently fold it in until the cereal is completely coated with the marshmallow mixture.

Step 5

Transfer to the pan. Transfer the mixture to an 8x8-inch baking pan. Using the rubber spatula, lightly and gently press into an even layer.

Step 6

Cool and cut the bars. Let the treats stand at room temperature until set, about 1 hour. Cut into 2-inch squares and serve.