Prepare a 9x9 pan by lining with parchment paper and spraying lightly with oil.

In a sauce pot combine the butterscotch chips with the peanut butter and butter. Heat over medium-low, stirring constantly until the butterscotch chips have melted and the mixture is smooth.

Add the salt and vanilla, mix to combine.

Set about 1 cup of the marshmallows aside, then add the remaining marshmallows to the butterscotch mixture and fold to combine.

Press the mixture into the prepared 9x9 pan firmly.

Cover the top with the cup of reserved marshmallows, gently pat them down to ensure they stick to the bars.

Let chill in the refrigerator for at least 1 hour or until solid.

Cut into squares before serving. Store in an airtight container in the fridge for up to 1 week.