I cook a lot of Asian food, and my pantry is full of "secret" ingredients for authentic flavor, such as mirin, fish sauce, dark sesame oil, furikake, and this humble spice, which wields mysterious powers. Szechuan peppercorns are the dried husks that surround the seeds of the Chinese prickly ash tree. They're a key ingredient in (big surprise) Szechuan cooking, which is known for intensely spicy and garlicky flavors.
Szechuan peppercorns aren't spicy in and of themselves, though; they have a faint lemony flavor. What makes them special is a chemical they contain that causes a slight numbing of the lips and tongue. It sounds scary, but it's not like getting Novocaine at the dentist. It's a subtle tingle that intensifies the spicy sensations of hot peppers. I inherited a big bag when a Filipino friend moved away and left me a treasure trove of ingredients from her pantry. Previously, I'd only encountered Szechuan peppercorns as a component of five-spice powder, a mixture including anise that's excellent in Chinese sauces and stir-fries.
How to use Szechuan peppercorns? I've been using Szechuan peppercorns in non-Asian dishes recently, and they add a whole new dimension. If you're a spicy-food lover, toss a few in with the vegetables when making chicken soup (strain them out before serving, or lightly crush before adding and leave them in). The tingling sensation makes the soup feel spicy, but none of its delicate flavor is overpowered. If you add Szechuan peppercorns to spicy dishes like puttanesca or salsa, it'll make them seem even hotter without covering up the other tastes like extra peppers can.
Look for Szechuan peppercorns at Asian markets or spice stores, or online. Use sparingly; a little goes a long way. Szechuan peppercorn fun fact: The USDA banned the importation of Szechuan peppercorns until 2005 because they carried a plant disease that harms citrus trees. They now must be heated to kill this disease before they can enter the US.