Pre-heat the oven to 175°C (350ºF) and grease a large spring-form (mine is 26cm).
Slowly heat the chocolate and butter in a pan until just melted, then add the coffee (or milk, if using). Set to one side while you prepare the next step.
In a large bowl, whisk the sugars with the eggs, then add in the cooled chocolate mixture.
Combine the vanilla sugar, flour and cocoa and stir into the wet ingredients, mixing until just combined.
Pour into the prepared tin and bake for around 20 minutes. The brownie is ready when the top is spongey but firm to the touch. Set to one side to cool for a while.
Stir the cashews into the dulce de leche and spread evenly over the top of the brownie.
Melt the milk chocolate in a Bain Marie (double boiler) and then pour over the caramel, spreading it with a knife to achieve an even coverage. Place the cake in the fridge for at least four hours (preferably overnight) to set (please see note 3).
When ready to eat, cut the cake into slices. Because the topping is just milk chocolate, it will set really hard. Therefore, it's possible it might crack (mine did). To try to prevent this from happening, use a sharp knife that has been run under the hot tap.