Since the dim sum downtown has sunk in a slough of grease, dumpling-seeking Sinophiles have few alternatives. At first glance, Casa Imperial is an unlikely candidate: a baroque mansion loaded with gargantuan crystal chandeliers, ersatz British hunting scenes and musty brocaded draperies. It’s about as Chinese-looking as Casa Loma, but the dim sum is quite wonderful. Don’t try asking your server questions, because most of them seem to speak no English. They wear black and white maid’s uniforms and bring the likes of rich seafood dumpling in perfect chicken soup and har gow with big chunks of sweet shrimp. You can see the chunks of Chinese mushroom and tendrils of pea green through rice wrapper so delicate it’s translucent. Fine quality hot XO sauce makes steamed pork with small black mushrooms exciting, and little cuttlefish tentacles profit by the sweet complexity of five-spice powder.