Quite simply, it's a custom that dates back to the late 17th century. At that time, white sparkling wines were very sweet (even more than some of today’s sweet white wines) and chocolate was very bitter, as the recipe had not yet been perfected. These two polar-opposite flavours balanced each other well, and ever since then we have continued to serve them together. You may not know this, but sparkling wines are white wines that are given their fizz through the addition of carbon dioxide. The dry quality of the white wine covers up the sweetness of chocolate desserts, which is a real shame.
To ensure a good pairing, the first thing to find out is the cocoa content of your chocolate treats. The lower the content, the better it will work with sweet wines. But for those with 70% cocoa and above, you can choose a full-bodied red wine, such as a Merlot or Syrah, the acidity of which will reveal all the cocoa aromas. When you get to 80% and above, opt for a slightly softer red wine, but avoid overly sweet or dessert wines, which should be saved for your cheese plates. If you’re reluctant to give up your pairing of chocolate desserts with sparkling wines, a rosé will be a much better match, as it’s much more mellow and fruity. And if you swear by white wine, then go for... white chocolate! That’s easy to remember ;)