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A nice dark Flourless Chocolate Cake would be something I would suggest, served with fresh raspberries and whipped cream, and chocolate curls or shavings. It is decadent and rich, flourless does not mean light.
It's gotten "wows". It's something you see at expensive restaurants, but it's fairly easy to make, though it does involve some techniques like folding beaten egg whites. It would be great for a chocolate lover! There's tons of recipes if you search. It's not like a layer cake, you would just make one layer in a springform pan, then say dust with icing sugar, and cut and serve in wedges with a dollop of cream, dark chocolate shavings, rasberries or whatnot. It is an easy cake to transport and will hold well at room temperature (I dislike taking cakes to people's houses that need refrigeration, because when entertaining people often have bulging fridges as it is. just put the whipping cream in the fridge).
Another "sick" chocolate one I do is a white and dark chocolate and raspberry ganache tart, with a chocolate crust. Ganache is that soft chocolate stuff in the middle of a truffle, made from chocolate and heavy cream and whatever else. The white and dark ganaches are sort of swirled in there with good quality fresh raspberries. I actually think that one's too "sick" for after Thanksgiving or Christmas dinner.
But thinking of that brought to mind that you could try making a box of truffles for her too as a little gift, they are easy to make, not expensive compared to buying them. Lots of recipes for those too on-line.