There is not much I can add that previous reviewers have not said (the reviews have been glowing!) but I can confirm that this place is a must visit for anyone visiting Madrid. Every time I need a gift for a chocolate lover this is the only place I go. Harvey is friendly, passionate, and energetic about his chocolate and he's always happy to give you a sample. The combinations he puts together are very unusual but absolutely amazing. My favorite so far has been the Green Chile Cashews and Macadamia Nut Chocolate he makes. Also, I had no idea that salted chocolate was soooooo delicious! This is a great souvenir to bring home for yourself or friends if you're visiting NM, you can guarantee that it will not go unappreciated!
Also check out the local handmade pottery available for sale in his shop while you're there, it's very nice :)