Grease and line a 20cm square cake tin and put aside. Prepare the chocolate base by putting the butter, cocoa powder and brown sugar in a saucepan, and heat till the butter has melted and the sugar has dissolved. Take off the heat and cool slightly. Beat in the eggs, and finally mix in the flour. Spread the mixture into the bottom of the cake tin and level out. Bake for 20 minutes at 160`c. Meanwhile start preparing the caramel filling. Melt the butter in another saucepan, and then stir in the condensed milk, golden syrup and sugar. Turn down the heat and leave to cook, stirring constantly, for 10-12 minutes. It is important to continue stirring otherwise the condensed milk will stick to the bottom of the pan and burn. When the caramel is thick and golden brown, take the saucepan off the heat and mix in the toasted hazelnuts. Immediately pour the hazelnut caramel over the chocolate base and even out with a palette knife. Leave to cool for an hour, or until the caramel to set completely. Next, pour the melted chocolate over the set caramel, spreading out all over. When the chocolate has set, take the cake out of the tin and cut into small squares or bars.