Preheat the oven to 150°C.

Grease and line a 20x30 cm baking tin.

Using an electric mixer, cream the butter, icing sugar and vanilla until light and fluffy. Add the egg and beat until combined. Sift over the cocoa, flour and salt and mix until just combined.

Press evenly and firmly into the base of the baking tin. Bake for 15 minutes.

Meanwhile, make the caramel. Combine the sweetened condensed milk, golden syrup, and butter in a medium-sized pot over low heat until melted and fully combined. Add the instant coffee to taste. Stir in the hazelnuts.

When the base is out of the oven, pour over the caramel layer and spread right to the edges. Return to the oven for anther 15 minutes or until set when you touch the surface.

For the topping, place the chocolate and butter in a heatproof bowl over a pot of simmering water. Stir until just melted.

Pour over the caramel layer, spreading right to the edges and smoothing the surface. Place in the fridge to set for 2-3 hours.

Once chocolate is set, slice up using a sharp knife dipped in very hot water (to prevent cracks in the chocolate layer). Sprinkle a bit of coarsely ground sea salt over the surface of the chocolate.

Keep in the fridge for up to a week (but it will probably be all gone by then!).