Translation:We are eating cake and drinking coffee.
"kakku" is "a/the cake"
"kakkua" is the word in partitive, so it can be translated as "cake" as well as "a/the cake"
"kakun" would be the word in either genitive (a/the cake's) or genitive-like accusative (a/the cake)
Since English doesn't have a similar case system, it's always a bit difficult to say which translation is the best one, especially when there is very little or no context given. "A cake" could be translated in many different ways. :)