Combine 3 cups flour, sugar, baking powder, and yeast in a large mixing bowl. Add warm water and ¼ cup vegetable oil. Beat with an electric mixer, fitted with a dough hook, on medium 2 minutes. Scrape sides of bowl and continue beating on medium 4 to 6 minutes or until mixture forms a soft, smooth dough. If dough is still sticky, add 1 Tbsp. flour at a time until dough forms a ball.

Turn dough out onto a lightly floured surface; knead dough 2 to 3 minutes. Lightly grease a medium bowl with oil; transfer dough to prepared bowl. Cover with a clean kitchen towel. Let rise in a warm place 1 to 1½ hours or until dough has doubled in size.

Turn out dough onto a lightly floured surface. Roll dough to ½-inch thickness. Cut into rounds using a 2½-inch round biscuit cutter. Reroll dough once to cut out additional rounds, if desired.

Flatten 14 (2½-inch) muffin cup paper liners on baking sheets or trays; set aside. Gently roll each dough round to a larger 4-inch round. Spoon 1½ tablespoon desired filling (such as: Sweet Red Bean, Korean BBQ Pancetta, or Sweet-and Spicy Filling) into center of each round. Fold and pleat dough around the filling; pinch top to seal. Place buns on flattened paper liners with pleated sides up. Let rest in a warm place 30 minutes or until dough is slightly puffy.

Fill a large saucepan with 1 to 2 inches water. Place steamer basket of same diameter on top of saucepan, making sure water does not touch basket. Bring water to boil over medium heat. Working in batches, transfer buns from paper liners to bottom of the steamer basket, placing 1 to 2 inches apart. Cover; steam 10 to 12 minutes or until buns are light and airy. Add additional water to the saucepan as needed.