In a medium saucepan, combine broth, tomato sauce, vinegar, syrup, garlic, Worcestershire sauce, mustard and pepper flakes; bring to a boil over medium-high heat. Reduce the heat to low and add chicken; cover and simmer for 30 minutes.

Using a slotted spoon, transfer chicken to a large bowl to cool for about 10 minutes. Continue cooking sauce, uncovered, until reduced by half, 10-15 minutes (you should have at least one cup). Remove from heat and transfer 1/2 cup of sauce to a small bowl, leaving remaining sauce in the pan.

Using 2 forks, shred (pull) chicken. Return pulled chicken to the saucepan and stir to coat evenly with remaining sauce.

(if making ahead refrigerate at this point)

Cook over low heat, just to warm through. Divide meat among buns and drizzle evenly with reserved sauce. Serve warm.