1) Preheat oven to 180C/Gas 4.
2) Lightly grease a 33 by 23cm baking pan. In a large bowl, combine the cake mix, 1 egg, and 115g melted butter, and stir until well blended. Pat mixture into prepared pan and set aside. In a stand mixer, or with a hand mixer, beat the cream cheese until smooth. Add the remaining 2 eggs, and the cocoa powder. Lower the speed of the mixer, and add the icing sugar. Continue beating until ingredients are well mixed. Slowly add the remaining 110g of melted butter, and the vanilla, continuing to beat the mixture until smooth. Stir in nuts with a rubber spatula. Spread filling over cake mixture in pan.
3) Bake for 40 to 50 mins. Be careful not to overcook the cake; the centre should still be a little gooey when finished baking. Let cake partially cool on a wire rack before cutting into pieces.