Line a 9 x 9–inch pan with aluminum foil or parchment paper, leaving an overhang on all sides. Set it aside.
Place the butter in a light-colored skillet. (The light-colored skillet will help you determine when the butter begins browning.) Melt the butter over medium heat, stirring constantly. Once melted, the butter will start to foam. Keep stirring. After 5 to 8 minutes, the butter will begin browning; you’ll notice lightly browned specks begin to form at the bottom of the pan, and you’ll start to smell a nutty aroma. Once browned, immediately remove from the heat, then 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 about ¾ of the sprinkles 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. Sprinkle the remaining sprinkles on top. 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 1 hour, before cutting into bars.
The treats can be stored in an airtight container at room temperature for up to 5 days.