Line a 9x13-inch (23x33 cm) baking dish with greaseproof paper.

Melt butter in a large saucepan over medium heat. Stir in butterscotch morsels until smooth.

Add marshmallows and cook until melted and creamy.

Stir in salt and vanilla, then remove from heat.

Add rice cereal and mix until evenly coated.

Transfer to the prepared dish and press down evenly.

Let set at room temperature for 45 minutes.

Drizzle with melted white chocolate, let harden, then cut into squares.