Line a 9x13-inch baking dish with foil and lightly spray with nonstick spray.
In a large pot over medium heat, melt the butter. Add butterscotch chips and stir until smooth.
Stir in marshmallows and cook until melted and creamy.
Mix in salt and vanilla, then remove from heat.
Add rice cereal and stir until evenly coated.
Transfer mixture into the prepared dish, then press evenly into the pan (lightly oiling your hands helps). Or use a spatula sprayed with non-stick spray.
Allow to set at room temperature for about 45 minutes.
Remove from the pan, drizzle with melted white chocolate, and let harden.
Slice into squares and serve.
Notes
Lightly greasing your hands or a spatula makes pressing the mixture into the pan easier and prevents sticking.
Store in an airtight container at room temperature for up to 3 days.