Stir together rice wine vinegar, soy sauce, chile garlic, hoisin sauce, brown sugar, corn starch and five spice in a shallow container. Add chicken and toss to coat. Cover and marinate in the refrigerator while making dough. 

Stir together 2 cups flour, corn starch, undissolved yeast, sugar and salt in a large bowl. Stir in warm water.  Turn out onto floured work surface and knead 4 to 6 minutes until smooth and elastic; adding additional flour if necessary. Cover and let rest 10 minutes.

Heat oil in nonstick skillet over medium-high heat. Remove chicken from marinade and add to skillet. Discard remaining marinade. Cook chicken 5 to 7 minutes or until cooked through. Remove from heat and spread on a plate to help cool. Chicken can be made ahead of time and kept in refrigerator for several days before filling buns.

Prepare a bamboo steamer: Cut 2 pieces of parchment paper to fit in steamer basket. Brush lightly with oil. Set aside.

Divide dough into 16 equal portions. Roll each portion into a 4-inch round. Place approximately 1 tablespoon chicken in the center. Bring up sides, pinch to seal. The seam can go on the bottom OR make a more decorative look by putting the seams on the top. Place roll on parchment paper. Repeat with remaining dough; leaving enough space between buns for rising. Depending upon size of steamer, these made be made in multiple batches. A typical bamboo steamer will hold 8 buns.

When steamer is ready, place parchment paper with bun into basket. Cover and steam 12 to 15 minutes or until dough is cooked through. Carefully remove lid; sprinkle with sesame seeds and green onions, if desired.

Remove buns from steamer using tongs. Discard parchment paper; repeat with remaining buns.

Serve with dipping sauce of your choice.