When I moved to London in 2005 I discovered The Chocolate Society in Shepherd Market, Mayfair. A great location, especially during the warmer months when you can sit outside in the sunshine. Apart from delicious chocolate truffles, they offer a variety of delicacies. I particularly recommend the dark chocolate brownies -- perfect in taste and consistency, one of the best I've ever tasted.
The Chocolate Society can also be found on Elizabeth Street.
phone number: 020 7259 9222