Sweet red wine or Port
The wine should be at least as sweet as the dessert to stay balanced.
Adapt the match to the dish
A very sweet dessert makes a dry wine seem more bitter: choose wine at least as sweet, then match cocoa intensity.
Wines to compare
For desserts, first compare the sweetness of the dish and wine, then look for enough acidity to avoid a heavy finish. Understanding sweetness in wine ↗
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The tip. With very dark chocolate, try a complex fortified red.