Preheat the oven to 160C Fan/180C, and grease and line a 23cm square cake tin.
Melt the chocolate and butter together in a pan over a low heat, making sure they don't burn at all.
Mix the eggs and caster sugar together until they are light and combined, add the coffee and mix evenly.
Add the melted chocolate and butter to the egg mixture and stir to combine.
Sift in the flour and fold gently making sure all of the flour is mixed in, add the hazelnuts, give it a final stir and place it all in the prepared pan and back for 20 minutes or until a toothpick comes out clean when inserted into the cake.
Leave the cake to cool in the tin, when the cake is completely cool, peel off the greaseproof paper and stand the cake on a board or cake rack.
Spread the chestnut spread over the top of the cake in a thin even layer.
Take the chocolate and put in it a bowl and stand it over a pan of barely simmering water.
Melt the chocolate, but don't let it get too hot, stir in the oil until the two have mixed together.
Take the chocolate mix and pour it over the top of the cake, spread it thinly with a knife and then place the cake in the fridge for around 15mins for the chocolate to set, so that the cake can be cut.
When the chocolate topping has set, trim the edge of the cake and then cut it neatly into 16 squares.