Sichuan peppercorn is essential for this dish. It cannot be replaced with other types of peppercorns. There are two types of Sichuan peppercorn, the red and the green type. Either works. An alternative to whole Sichuan peppercorn is to use store bought ground Sichuan pepper (may also known as Chinese pepper powder 花椒粉), which is more common in grocery stores. It won't be as fragrant as using whole peppercorns, but will still have the distinctive numbing taste of Sichuan peppercorn.