Line a 9x13-inch pan with foil and coat with nonstick spray. Set aside so it's ready when the mixture comes together quickly.
In a large pot over medium-low heat, melt the butter. Keep it low so it doesn't brown or scorch.
Stir in the vanilla and salt once the butter is melted. This helps distribute the flavor before adding the marshmallows.
Add 1½ bags of the marshmallows to the pot. Stir constantly until they’re completely melted and smooth.
Take the pot off the heat. Add the remaining ½ bag of mini marshmallows and 1½ cups of the crushed Oreos. Stir to gently combine.
Pour in the Rice Krispies and stir until everything is evenly coated. Work fairly quickly before it cools.
Transfer the mixture into the prepared pan. Use a greased spatula (lightly buttered) to press it down evenly.
Let the bars cool completely at room temperature.
Microwave the white chocolate in a heatproof bowl in 20 - 30 second bursts, stirring in between, until smooth.
Drizzle the melted white chocolate over the bars, then sprinkle with the remaining crushed Oreos. Let everything set before slicing.
Store at room temperature in an airtight container for up to 3 days.
