Prepare your pan by lining it with parchment paper and lightly spraying with cooking spray. This prevents sticking and makes for easy removal.
Melt the butter in a large pot or Dutch oven over medium-low heat until completely melted. Watch carefully to prevent browning.
Add most of the marshmallows (except the reserved 2 cups) to the melted butter and reduce heat to low. Stir constantly until marshmallows are almost completely melted with just a few small lumps remaining.
Remove from heat and immediately stir in the vanilla extract and salt. The residual heat will continue melting the marshmallows while these ingredients enhance the flavor.
Add the Rice Krispies cereal to the marshmallow mixture and gently fold until just combined. Don’t overmix or compress the cereal, as this will result in dense treats.
Fold in the reserved marshmallows (and sprinkles if using) while the mixture is still warm but not hot. These will create pockets of extra gooey goodness throughout your treats.
Transfer the mixture to your prepared pan and gently press into an even layer using a piece of parchment paper or buttered hands. Press just enough to even out the surface without compacting.
Let cool completely at room temperature for about 30 minutes before cutting into squares with a sharp knife.