Sponge Cake: Preheat oven to 160 degrees C. Grease cake tin(s) well. 1 large tin with cake cut in half or 2 smaller tins.
Sieve flour, cocoa powder, sugar, bicarbonate of soda and salt together in a large bowl. Mix well.
Add oil and sour cream then mix them in very well so that all the dry mixture is now moist and a dark chocolaty colour.
In a small bowl mix water, vinegar and vanilla. Add this bowl to large bowl's mixture and continue to stir.
When the mixture looks even beat the eggs very slowly; bit by bit to the batter stirring the whole time.
Before pouring into cake tins add chocolate chunks and stir through evenly. Put into oven and let it bake for 30-35 minutes. Try not to open the oven during this time or it will sink.
When baked allow it to cool in the tin for about 20 or 30 minutes and then carefully turn it out; careful as it is very soft and moist. Let it sit for another 10 to 20 minutes.
Icing: Beat with an electric mixer the butter and cream cheese then add the sugar when it looks even. Beat beat beat. Add peanut butter. Continue to beat.
Hard Chocolate Icing: Melt chocolate and butter in a bowl over boiling water. Be careful not to allow any water/condensation in. When the chocolate is quickly and successfully melting take off the heat and add the golden syrup. It should be very melty and sticky.
To Assemble: Spread the icing thinly and evenly over cakes. Place in the fridge for 5 minutes, then richly, thickly cover the cake with the remaining icing. Return the iced cakes to the fridge uncovered.
While the hard chocolate icing is still gooey pour it all over the cake. Put back in fridge. The chocolate should harden, or you could eat while it is still hot.