The Shulchan Aruch O'C' 318' 5' discusses an argument in between the Poskim whether or not there is 'Bishul' (cooking) after 'Tselee' (roasting) or 'Afiyah' (baking).
However,as the Mishnah Berurah points out in Se'eef Koton 41', according to all Poskim there is no Tselee after Tselee. Afiyah is equivalent to Tselee as they are both dry. Therefore we conclude that there is no Afiyah after Afiyah either.
[Now i add, that altough the Shlchan Aruch does not mention Afia specifically, the Tur however mentions it out right as being equal to Tselee.]
Regarding delicious Chocolate Melt away cake; in Se'eef 16' the Shulchan Aruch discusses a similar idea - Bread with a filling of fat inside, which contains 2 potential problems with reheating it. One being Bishul and the other 'Nolad' which means creating something new.
The Mechaber allows it and the Mishnah Berurah explains that since before reheating it, it is dry there is no Bishul anymore although when reheated it shall be soft.
Regarding the issue of Nolad, it is permitted being that he does not melt it with his bare hands. However the Re'Ma brings down those that are stringent in the aspect of Nolad. They claim that since when it melts it will flow out and will be noticeable by itself there will be a problem of Nolad.
However, says the Mishnah Berurah, when there isn't that much fat and only a little will flow out it shall be allowed according to all opinions for that would not be considered an entity in itself in order for it to go into Nolad.
The Mishnah Berurah says as well that heating up a piece of Chicken which contains some fat on top of it will be permitted since it is a small amount it is not Choshuv (significant), and would be permitted.
Considering our Cake, if the Chocolate right now is not in a liquid state it shall not be Bishul although it will melt upon reheating.
Nolad, it will not either be since there is no vast amount of Chocolate which would bring a lot of liquid to flow out upon heating, but rather just relatively small amount will flow out and being its not a significant amount, it won't be considered its own new entity to be regarded as Nolad.
Having said all this, it would be permitted to reheat the cake in a place next to fire even when it will heat it up to the extent that it will be 'yad soledes bo' meaning that your hand will hurt a significant amount after keeping your hand on it for a couple of seconds.
However on the fire one may not put it, as the halacha states in siman 253' that mi'derabanan, because of the Chacamim's fear that one will end up playing around with the fire in a way that will be considered Havarah which is the Melacha of lighting a fire and that includes placing it on the blech right on top of the fire which is as good as placing it on the fire directly.
The blech helps only to allow one to return something that was on the fire on Shabbos and that is called chazarah.
Therefore reheating the cake will be permitted when its not directly on the fire as the Shulchan Aruch calls it 'keneged ha'medurah'.
Another way to reheat the cake would be to use an electric hot-plate (plata). There are differing opinions as to whether one can place cold, cooked, dry food directly onto the plata, on an upturned (empty) pan on the plata, or on top of another pot cooking on the plata. See the answers here for details.
However, as far as this all relates to the original question (reheating the cake in an oven) it is generally assumed that one cannot reheat cold, cooked, dry food in an oven which is still on (even if the oven has a "shabbat" mode). See here. This is presumably equivalent to 'placing the food directly on the fire', as mentioned above.