1.Preheat the oven to moderately slow (160°C). Grease a 20cm x 30cm lamington pan, line the base and two sides with baking paper.
2.Combine the butter and cocoa in a medium saucepan, stir over low heat until smooth. Add sugar, stir until dissolved.
3.Remove from heat, add vanilla, eggs and sifted flour; mix well. Spread mixture into prepared pan, bake in a moderately slow oven for 20 minutes; cool.
4.For the caramel filling, combine butter, sugar, syrup and condensed milk in a medium saucepan; stir over a low heat until butter is melted. Increase the heat to medium and simmer, stirring, for about 13 minutes or until mixture is a dark caramel colour.
5.Remove from the heat and stir in the hazelnuts. Quickly spread the caramel filling evenly over the base, refrigerate for at least 30 minutes or until firm.
6.Combine the chocolate and oil in a small bowl then spread over the caramel filling; refrigerate until set.