In a large pot, melt butter over medium heat. Reduce heat to low and add marshmallows. Add vanilla, salt, and cereal. Remove from heat and stir to coat.
Scoop mixture out onto a large parchment lined cookie sheet. Even out the edges and center to form a rectangular shape. Let stand for 10-15 minutes.
Warm butterscotch for 20 seconds in microwave. Drizzle slowly over rice crispies and top with chopped chocolate. Cut into squares and serve.
Store in an air tight container for up to 5 days after preparation. Store in fridge for longer.