Line a 9x5–inch loaf pan with aluminum foil or parchment paper, leaving an overhang on the two long sides. Spray the pan lightly with nonstick cooking spray and then set it aside.
Place the butter in a large saucepan and melt it over medium heat, stirring constantly. Once melted, remove the pan from the heat and stir in the salt and vanilla.
Working quickly, add half of 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 without adding the heat, but you can return the pot to medium-low heat for a few minutes if the second addition of marshmallows isn’t melting quickly.)Note: After you add the marshmallows, keep the heat on low or below medium-low. If your marshmallow is exposed to heat that’s too high, you’ll end up with treats hard enough to break your teeth!
Add the cereal and gently fold it in with a rubber spatula coated in nonstick cooking spray until the cereal is completely covered with the marshmallow mixture. Transfer the mixture to the prepared pan. Using the rubber spatula, gently press the mixture into an even layer. Note: Don’t pack the Rice Krispie Treats into the pan too tightly. If you pack them too tightly, they can end up dense and difficult to bite through.
Let the treats stand at room temperature until set, about 30 minutes, before cutting into bars.
The treats can be stored in an airtight container at room temperature for up to 5 days.