Add sugar to a saucepan over a medium/high heat, constantly stirring until the sugar has melted (this should take approx. 5 mins — note that it will clump up before turning smooth).
When the sugar is smooth and amber in colour, lower the heat, add the butter and whisk. If the caramel clumps up, the butter is still too cold — to counter this: turn the heat back up and whisk until the sugar crystals have melted again.
Once the butter is fully combined, slowly pour in the double cream whilst continuing to whisk — at this point it will bubble up slightly.
Leave to boil for 1 minute, then add the vanilla and salt, and allow to cool.
Add all milkshake ingredients to a blender and blend until smooth.
Decorate the inside of 4 regular-sized glasses with salted caramel sauce, add the milkshake, top with the little sprinkle of MALDON SALT and serve.