Line a 9x13-inch baking pan with aluminum foil and spray with nonstick cooking spray.
Melt the butter in a large saucepan over medium-low heat. Once melted, stir in the vanilla and salt.
Add 10 ounces of mini marshmallows to the melted butter and stir frequently until fully melted.
Remove the pan from the heat and add the rice crispy cereal. Stir until well incorporated with the marshmallow mixture.
Add the remaining marshmallows and rainbow sprinkles to the pan. Fold them into the mixture until well incorporated.
Transfer the mixture to the prepared pan and press down using a rubber spatula to form an even layer.
Allow the treats to cool at room temperature.
In a microwave-safe bowl, melt the white chocolate in 20 to 30 second intervals, stirring between each interval, until smooth.
Remove the bars from the pan and set on a cutting board or clean surface. Drizzle with the melted white chocolate. Immediately top with the crushed oreos, extra sprinkles, and extra marshmallows.
Let the white chocolate harden before slicing into bars.
Notes
Store in an airtight container at room temperature for up to 3 days.