For a little extra flavor, marinate the chicken and shrimp in a little sake and/or soy sauce for about 10 minutes, then drain and pat dry before adding to the cups. Chawanmushi cups with lids are relatively easy to find online or at Japanese or Asian grocery stores, but you can use any heatproof vessel that is basically the same width at the top and bottom, such as custard cups, ramekins or teacups. The timing may differ, though, depending on the size, shape and material, so do a test run with one chawanmushi first. Also, if you can’t fit all the cups in the steamer at once, cook them in batches.