Melt the butter in a large saucepan over a medium heat, then add the cream and brown sugar. Stir everything together, then leave alone as you bring it to a boil.
Check the temperature periodically - you want it to reach 110c to be the perfect consistency.(f you don’t have a sugar thermometer, boiling it for about 4 minutes will give you a similar result.
Remove from the heat, and stir in the salt, vanilla extract, and dark rum, then set aside to cool.You can speed up cooling by placing the bottom of the saucepan into cold water.
Preheat the oven to 180c / 160c fan.Grease and line a 9”x13” tin with baking parchment.
Set up a Bain-Marie over a low heat, and gently melt the butter and chocolate together.Stir occasionally with a spatula, and once smooth set aside to cool slightly.
In a stand mixer fitted with a paddle attachment, combine the caster sugar, brown sugar, salt, and vanilla. Mix together momentarily, then add the eggs one at a time, stirring well after each addition.
Add the chocolate mixture and mix until everything is well combined.
Lastly, add the plain flour, and mix just until you can no longer see dry patches.
Pour 3/4 of the brownie batter into the prepared tin, and tilt and shake to pan to create an even layer.
Pipe 2/3 of the butterscotch over the surface of the brownie batter in a diagonal pattern.
Add the remaining brownie batter, then shake and tilt the pan to cover evenly.
Finally, use the remaining butterscotch to create diagonal lines over the surface.
Bake for about 50 minutes, then leave to cool completely in the tin.
In order to cut the brownies neatly, chill completely in the fridge first.Serve either at room temperature, or warmed.