Place jackfruit in a deep skillet and cover with water. Bring to a boil over high heat then reduce heat and simmer, uncovered, for 15 minutes. Drain the water and add the Korean BBQ sauce. Simmer, stirring occasionally, for about 5 minutes or until the sauce thickens slightly and the jack fruit is coated.
Stir the mayonnaise and Sriracha together in a small bowl. Lightly toast the buns. Spread the spicy mayonnaise on both sides of the buns. Top with the jackfruit and lettuce plus any additional toppings desired.