Combine the sourdough starter, all-purpose flour, cornstarch, granulated sugar, salt, baking powder, olive oil, and lukewarm water in a mixing bowl.

Knead the dough until smooth and elastic.

Let the dough rise in a warm place until it doubles in size, approximately 1 hour.

Divide the dough into 12 equal portions and shape them into buns.

Place the buns on parchment paper and let them rest for about 30 minutes.

Steam the buns for 12 minutes until they are fluffy and cooked through.