Place the butter, brown sugar, vanilla extract and salt into a microwave-safe bowl. Heat in the microwave for 2-3 minutes in 30 second bursts (50% power) until the sugar has almost dissolved and the ingredients have combined - stirring each time.
Allow to cool slightly before adding the eggs and whisk until combined.
Sift the plain flour and baking powder into the bowl and gently fold through. Stir through the butterscotch (or Caramilk) chips, if using.
Sprinkle over the sea salt.Pour into the prepared baking tin. Bake for 20-25 minutes or until cooked through.