Preheat the oven to 180`c and line 2 x 20cm round tins. Start this recipe by mixing together the cocoa, dark brown sugar and boiling water then put aside. Next, beat together the softened butter caster sugar and vanilla essence till light. Sift together the flour, baking powder and bicarbonate. Add the eggs into the creamed butter, adding in a heaped tablespoon of the flour mixture with each egg. Mix in the remaining flour mixture followed by the cocoa mix prepared earlier. Spoon the mixture into the prepared tins and bake at 180`c for 30 minutes. Allow to cool down completely before frosting. Meanwhile, make the frosting by beating together the unsalted butter, icing sugar and vanilla till light and fluffy. Beat in the cream cheese. Spread ½ onto 1 of the cakes, then crumble over ½ of the oreo biscuits. Place the second cake on top, cover with the remaining frosting and crumble the remaining biscuits on top