How to Make It
Combine yeast, water, and 1/2 teaspoon sugar in the bowl of an electric mixer fitted with a hook attachment. Let yeast bloom until slightly foamy, about 5 minutes. Combine flour, cornstarch, remaining 5 tablespoons sugar, and baking powder, stirring with a whisk. Add to the yeast mixture with the oil and stir with the dough hook on low until all ingredients are fully incorporated, about 3 minutes. Knead gently on a clean work surface until smooth, about 5 turns. Place in a bowl coated with cooking spray and cover with a damp cloth; let sit in a warm place (about 90°F) until doubled in size, about 1 hour.
Heat 1 tablespoon sesame oil in a large skillet over medium high. Add shallot and cook, stirring constantly, until softened, about 1 minute. Add drained jackfruit and cook until beginning to crisp, about 5 minutes. Reduce heat to medium and add hoisin sauce, sugar, soy sauce, remaining 11/2 teaspoons sesame oil, and sriracha; cook, stirring frequently, until mixture comes to a simmer, about 1 minute. Remove from heat.
Cut a large piece of parchment into 10 (4x4-inch) squares; set aside. Bring 1 inch of water in your steamer basket to a boil.
Roll dough into a long log and cut into 10 equal portions. Flatten each portion into a 4 1/2-inch circle, making sure it is thinner around the edges and thicker in the middle. Add 11/2 tablespoons filling and enclose like a coin purse (4 corners, then remaining 4 corners).
Place each bun on a prepared parchment square and place in the steamer; steam over high until dough is puffed and spongy, about 10 minutes. Serve immediately.