Flavor – adjust flavor from inexpensive brandy

I haven't cooked chocolate with regular wine per se, but my instinct is that to get the right marriage of flavour you really might need to consider alternative approaches with both a higher sugar and alcohol content: dark chocolate is a very powerful flavour that will obliterate most wine.

So I would try using something else wine-related but with a considerable higher sugar and alcohol content: Port and chocolate certainly can complement each other well - I would recommend a tawny port (I have used this combination as a sauce for a savoury dish). You might try sherry too, but this is slightly riskier territory in my opinion.

A further alternative depending on the dish you are cooking would be to pursue your current course add a third component that will help with the overall flavour structure, typically something bittersweet and with fruit, such as redcurrant jelly.

It may have been the act of leaving the plums on top of the cake, covered overnight at room-temperature. This might have concentrated the natural ethylene produced by the plum enough to cause it to ripen by breaking down the starches and turning it into sugars.