Line a 18 X 28 cm (7 X 11 inch) shallow cake tin.
For the base: Melt the butter with the cocoa. Off the heat, stir the flour, sugar, nuts, essence and the eggs. When well mixed, spread the mixture in the tin.
For the topping: Beat the cream cheese with the sugar, flour, butter, egg and essence until smooth and almost white in colour. Spread this carefully over the base. Sprinkle grated chocolate on top, then bake at Gas Mark 4/180° for 45-55 minutes, until risen and cooked through. Cool in the tin; it will sink in the middle. Cut into bars. Lift out carefully.