Dissolve the yeast in 1/4 cup warm water. Add 1 teaspoon sugar, 1/4 cup flour, and stir to mix well. Allow to stand for 30 minutes.

Mix in remaining warm water, remaining flour and sugar, salt, and lard or vegetable oil. Knead until dough surface is smooth and elastic. Roll over in a greased bowl, and let stand until triple in size, about 2 1/2 to 3 hours.

Punch down dough, and spread out on a floured board. Sprinkle baking powder evenly on surface, and knead for 5 minutes. Divide dough into 2 parts, and place the piece you are not working with in a covered bowl. Form each half into a log about 2 inches thick. Cut logs into 1-inch pieces and roll each into a ball. Cover with a damp towel.

Working with 1 piece of dough at a time, press into a thin disc shape. Place about 1 tablespoon of filling in center, and then gather up edges and twist to seal, using some extra drops of water to stick together as needed. Cover with a damp kitchen towel and repeat with remaining dough.

Line bamboo steamer tray with lettuce or cabbage leaves. Place buns, sealed side down, in bamboo steamer tray and let rise for 45 minutes. Meanwhile, heat water to boiling in a wok to a level just below the bamboo steamer.

Place bamboo steamer in wok and steam buns for 20 to 25 minutes, and then remove from heat. Remove lid before turning off heat, or else water will drip back onto bun surface and produce yellowish "blister".

Place a wok over medium heat. Add the oil and, when hot, add the ginger, and green onion and stir-fry until aromatic, about 1 minute. Add the shrimp and bamboo shoots and cook for 1 minute. Add the sugar, soy sauce, and sesame oil and stir to combine. Combine the cornstarch and the water and add to the shrimp mixture, stirring frequently. The mixture should thicken immediately. Transfer mixture to a bowl and allow to cool before assembling the buns.